Alongside pre-orders for the Apple Watch and the Samsung Galaxy S6 and S6 edge phones, the Star Wars Digital HD collection went on sale today, marking the first time that the trilogies have been available in a purely digital format (bar illegally).

Digital HD is the format pioneered by 20th Century Fox and is adopted by several of the big name retailers, including iTunes, Amazon and Xbox, and it presents films in as high a 1080p quality as possible, with 5.1 surround sound, for a file size of around 5GB.

It is also now supported by so many digital lockers that you can build a decent-sized collection of Digital HD films. And this new Star Wars collection is ideal for those who want to take the movies on their travels, stored on their tablets, smartphones or laptops, without going through the laborious process of encoding digital versions of their own Blu-rays or DVDs.

However, we can't escape the feeling that releasing all six movies on yet another format has its issues too. So here are six reasons (one for every film in the collection) as to why we think you shouldn't bother buying them, even with new and unseen extras.

Video encoding technology has come on leaps and bounds in the last few years but regardless of how crisp the images look on a digital download version of a film, they will always be trumped by the Blu-ray version. That's because a Blu-ray has far greater storage space for video content: 25GB for a single-layer disc, 50GB for double-layer. A lot of that will be taken up by the audio encode, but the video will still be a mightily large file.

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As the digital version needs to either be downloaded or streamed, it needs considerably greater compression to fit the video into a manageable file size.

For example, the video file on the Star Wars: Episode V - Empire Strikes Back Blu-ray disc is a little under 40GB, while the Full HD Digital HD version is 5.29GB. And the latter includes the 5.1 Dolby surround track too.

The more compression there is on the video file, the more detail will be lost. So while the Digital HD versions look great, they don't look as great as the Blu-ray.

READ: Fox talks Digital HD: The UK invasion, Ultra-HD 4K, and why it could eventually replace Blu-ray

Yes, the Digital HD versions have the 5.1 Dolby Digital surround track found on the Blu-rays and DVDs, which will be fine for some, they don't include the greatly improved 6.1 DTS-HD mixes that come with the BD collection.

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That might not be important when you are out and about, but if you've spent a wedge on a home cinema setup, with a compatible AV receiver, you'll like your films to sound as good as they look. Only the Blu-ray offers the best option in that case.

READ: This is what it would look like if Star Wars and Final Fantasy collided

It must be said that the Digital HD Star Wars collection is very highly priced for back catalogue movies, even if this is their first appearance in a digital download format. £13.99 for the HD version of each film, £9.99 for the SD, is comparable to other brand new releases, including Interstellar and The Hobbit: The Battle for The Five Armies.

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Older, back catalogue releases cost anywhere from £6.99 for the SD version, £7.99 for the HD. Yes, we know these are Star Wars films and yes they have only technically been released in this format for the first time, but charging that much seems steep, even with the stack of extras - including some unseen bonus features.

The whole HD collection is £69.99, which is a large outlay too.

Star Wars: The Complete Saga on Blu-ray is currently £20 less, at £49.99.

READ: 11 funny alternatives to the ‘Star Wars: The Force Awakens’ trailer

When the movies were released on DVD for the first time, they were encoded from all-new 4K Ultra-HD digital restorations - of the original trilogy at least - which means that 4K masters of all six films are out there. And like night follows day, Disney and Fox will be waiting until the format matures a little more before unleashing yet another collection of the trilogies - this time in Ultra-HD.

It might be worth waiting for those therefore, especially if you already own the Blu-ray box-set, as they will offer something considerably better than you have already. And they will probably be accompanied by even more unseen extras that were strangely never unearthed for any prior releases.

Although the timing of the Digital HD releases do make it possible for newcomers to catch up with the films on multiple devices prior to the release of Episode VII, they can only do so by buying the individual digital versions outright, rather than rent them.

Surely offering rentals of each would allow them (or others who wish to catch up) an opportunity to see them once each at a lower cost? You can rent just about every other film on any of the services currently offering the collection.

And finally, one of the things we think would have made the collection's steep price tag a little more worthwhile and attractive would have been an exclusive, extended trailer for the seventh movie, coming from Disney later this year.

Instead, it is heavily rumoured that the new trailer will accompany Avengers 2 on its release in the US on 1 May. That's hardly reward for the Star Wars fans willing to part with even more cash for the Star Wars trilogies in yet another new format.