Fitbit offers several devices in the activity tracking market, from the plain and simple Flex to its GPS sportswatch, known to friends as Surge.

The company has recently announced another couple of devices to its line-up however, with both a Flex successor and a Charge HR successor arriving before the end of the year.

We've compared the Charge 2 to the Charge and Charge HR to see how the devices differ and help you work out whether or not you should upgrade.

The Fitbit Charge and Charge HR both measure 21mm wide and they both come with a small OLED display and a textured, elastomer band. There is a small button on the left-hand side of the display, they both come in several colour options and they are both water resistant up to 1ATM but neither are swim proof.

The non-removeable strap is fastened with a stainless steel clasp on the Charge HR and two pins on the Charge. The Charge HR also features Fitbit's PurePulse Heart Rate monitor on its underside, while the Charge doesn't.

The Charge 2 offers a similar strap and clasp to the Charge HR, made from the same flexible elastomer material with a stainless steel buckle and measuring 21mm in width. The strap on the Charge 2 is interchangeable however, featuring the same mechanism as the Fitbit Alta and allowing users to switch between classic and leather bands that are sold separately.

The display is much larger on the Charge 2, five times larger in fact, taking over the majority of the top of the device. There is also a larger button on the left of the display and a stainless steel body underneath, again like the Alta.

The PurePulse Heart Rate Monitor is featured on the underside of the Charge 2 like the Charge HR, and we'd there are four main colour options, with two special addition options. The Charge 2 is sweat, rain and splash proof like its predecessors, but not swim-proof.

Both the Charge and Charge HR track steps, distance, calories burned, floors climbed, active minutes, hourly activity and stationary time. They also both allow the user to see daily stats and the time on the small display, along with incoming call notifications.

The Charge and Charge HR are capable of sleep monitoring and they both feature silent alarms. Additionally, they both have SmartTrack on board, which automatically recognises when you exercise and records it in the app.

The Charge HR differs slightly from the Charge in that it has the PurePulse Heart Rate monitor on board, as we mentioned. This gives users an automatic and continuous heart rate reading from their wrist.

The Charge 2 is features all-day activity and sleep monitoring, meaning all the same features as the Charge and Charge HR. It also offers Multi-Sport tracking, which is a feature that was introduced on the Blaze earlier this year and allows users to track one of seven activities from walking to yoga. The PurePulse Heart Rate monitor is also on board, as we've said, meaning the new device is more of a successor to the Charge HR than the Charge.

The Charge 2 will also deliver smartphone notifications, like the Alta, which means users will see calendar alerts, text messages and call notifications appear but no third party notifications. Additionally, the Charge 2 features Connected GPS, a new feature called Cardio Fitness Level that allows users to see a personalised Cardio score and another new feature called Guided Breathing. This latter feature offers personalised breathing sessions based on your heart rate.

The Fitbit Charge 2 offers a similar design to the Fitbit Alta but on a larger scale and with several extra features.

What does the Charge 2 bring over its predecessors? Well, it adds smartphone notifications, Multi-Sport tracking, Connected GPS, Cardio Fitness Level and Guided Breathing over the Charge HR and Charge HR, as well as a bigger display, interchangeable straps and a more premium, streamlined design.

The Charge 2 also offers heart rate monitoring, which although the Charge HR offers this, the Charge doesn't and neither does the Alta. Ultimately, the Charge 2 appears to be a bigger and better version of the Fitbit Alta.

The Charge 2 is available to pre-order now for £129.99 or $149.99. It will ship from mid-September.