OnePlus introduced two smartphones in 2015 to deliver a now three-strong portfolio. The second "flagship killer" OnePlus 2 launched at the end July, followed by the cheaper and smaller OnePlus X at the end of October.

The OnePlus X is brings with it some pretty decent specs for sub-£200, but so does the OnePlus 2 for a £239 starting price and the OnePlus One, which was excellent at its time of launch is still available, although out of stock at the time of writing, with the bonus of a lower price at £179. The question is how do they all differ from each other and how do all three OnePlus devices compare?

We have put the OnePlus X up against the OnePlus 2 and the OnePlus One to see what the differences are and help you decide which device is the best one for you.

The OnePlus X measures 140 x 69 x 6.9mm, which is the smallest and slimmest smartphone within the OnePlus range. By comparison, the OnePlus 2 measures 151.8 x 74.9 x 9.85mm and the OnePlus One 152.9 x 75.9 x 8.9mm.

The original OnePlus One comes in Silk White and Sandstone Black colour options, both of which weigh 162g. It is now only available in the white though and as we said, that was out of stock at the time of writing. The OnePlus 2 comes in Sandstone Black only, weighing 175g, but there are four covers you can add to get a slightly different look.

The OnePlus X takes a different route with Onyx and Ceramic models available. The Oynx model features black glass and a metal bezel and hits the scales at 138g, making it the lightest OnePlus smartphone. The Ceramic model is limited edition, with only 10,000 up for grabs, and it weighs 160g so still lighter than the OnePlus 2 and OnePlus One.

Neither the OnePlus X or OnePlus One come with a fingerprint scanner or USB Type-C, while the OnePlus 2 offers both.

The OnePlus One and the OnePlus 2 both offer wonderfully solid designs that we were big fans of when we reviewed them. With the OnePlus X, we felt that while it appears every bit the premium handset in photos, it felt a little cheap in the hand. With this in mind, we would opt for the OnePlus 2 out of all three when it comes to design.

The OnePlus X has a 5-inch Full HD display, which means a pixel density of 441ppi. It also means this device has the smallest display of the OnePlus crew.

The OnePlus 2 and OnePlus One both offer 5.5-inch displays. As the pixel resolutions are the same as the OnePlus X at 1920 x 1080 however, the OnePlus X offers the sharpest display of the bunch, with the other two devices offering a pixel density of 401ppi.

Numbers are numbers but in reality, all three of the OnePlus smartphones offer excellent displays. The OnePlus One and OnePlus 2 have big, sharp and colour-accurate displays, while the OnePlus X has a screen that pops thanks the rich, deep colours and inky blacks. You won't be disappointed with any of the displays.

The OnePlus X features a 13-megapixel rear camera with an f/2.2 aperture and 0.2-second autofocus. There are several functions on board, including HDR and Clear Image, while the 8-megapixel front snapper has Beauty Mode to deliver pretty selfies.

The OnePlus 2 also has a 13-megapixel rear camera, as does the OnePlus One, but the aperture on the OnePlus 2 is slightly wider at f/2.0. The OnePlus 2 also has 0.2-second autofocus and optical image stabilisation, which the OnePlus X does not. 

The OnePlus 2 and OnePlus One both have a few less megapixels on the front compared to the OnePlus X, with 5-megapixel snappers running the show.

In terms of performance, the OnePlus X has a fairly average camera in our experience. Images exhibited a notable amount of noise and it wasn't as proficient as the OnePlus 2's camera. On that note, the OnePlus 2 isn't as strong in operation as the likes of the Samsung Galaxy S6 but it does a sterling job when it comes to image quality and the OnePlus One's is pretty good too. So all-in-all, for the best camera performer, you'll want to look at the OnePlus 2 or OnePlus One if your budget is less accommodating.

The OnePlus X features the Qualcomm Snapdragon 801 processor with Adreno 330 graphics, supported by 3GB of RAM. This is the same hardware as the OnePlus One, while the OnePlus 2 bumps it up slightly to the Qualcomm Snapdragon 810, Adreno 430 graphics and 3GB or 4GB of RAM.

There is a 2525mAh battery powering the OnePlus X, while the OnePlus 2 has a 3300mAh capacity and the OnePlus One features a 3100mAh.

In terms of storage, the OnePlus X has 16GB of internal memory with microSD support for storage expansion up to 128GB. The OnePlus 2 comes in 32GB and 64GB options, while the OnePlus One did come in 16GB and 64GB options, but the 64GB model is no longer available. Neither the OnePlus 2 or OnePlus One offers microSD expansion.

We were more impressed with the performance of the OnePlus One and the OnePlus 2 than the OnePlus X with the latter facing a few issues in some areas, specifically more graphically intensive tasks. In terms of battery, the OnePlus One gave us around a day and a half, while the OnePlus 2 offered a little over a day and the OnePlus X around 17 hours.

The OnePlus X comes with the company's own OxygenOS 2, which has been developed on Android 5.1.1. This is the same operating system as the OnePlus 2 so you'll get the same user experience on both devices.

The platform offers an Android experience with enhancements, such as off-screen gestures and customisable buttons. There are features such as Dark Mode and you'll also be able to do things like change the accent colour.  You can read more about Oxygen in our separate feature here. Overall, OxygenOS feels a lot like Android but it offers some genuine benefits like the SwiftKey Keyboard from the off.

The OnePlus One comes with Cyanogen Mod, which is also built on Android. It is possible to manually install Oxygen but it doesn't come as standard. We got on well with both Cyanogen and Oxygen but some will prefer vanilla Android, despite some of the useful customisations.

The OnePlus X Oynx costs £199, while the Ceramic model costs £269 and both require an invite. The OnePlus 2 starts at £239 and jumps to £289 for the 64GB, which is the only model currently available not he OnePlus site. The OnePlus One costs £179, although it is currently out of stock.

Therefore, the OnePlus One is the cheapest way to get your hands on a OnePLus smartphone, but as it is out of stock, is also hard. The OnePlus X of course requires an invite so that's no easier than the OnePlus One, while the OnePlus 2 is the most expensive but currently the easier to buy.

The OnePlus X is the smallest, slimmest and lightest OnePlus smartphone available. It also offers the sharpest display, albeit also the smallest and the most megapixels in terms of front camera.

The OnePlus 2 wins the hardware category with the fastest processor and largest battery capacity, but the OnePlus X offers microSD expansion, which the others do not. You also get the latest software on the OnePlus X as standard so you'll get a very similar user experience to the OnePlus 2 but for £90 less if you opt for the Oynx model. 

The OnePlus X is for those after decent specs at a good price, along with the OnePlus experience but in an overall smaller package. It isn't the OnePlus smartphone we would choose first however as it's rocky performance and cheaper-feeling build would steer us towards the extra bucks for the OnePlus 2. The OnePlus One isn't a bad option for those that have a tighter budget, or no invite, but if you can find the extra cash, the OnePlus 2 is the overall winner here.