UPDATE: THE STATEMENTS IN THE ORIGINAL POST ARE THANKFULLY NO LONGER RELEVANT
As of May 3, 2016, Disney has updated their Steam entries for X-Wing and Tie Fighter to include the sorely lacking Collector's CD-ROM editions (from 1994/1995). So it's not longer undesirable to acquire the Steam versions as opposed to the GOG versions!
The GOG versions still do come with bonus extras like strategy guides and whatnot, but the all the game versions themselves are included at both sources now. So game on any way you like!
(For the DOS versions you can still play DRM-free since they are DOS games and can't be tied to Steam. The same might be true for the 1998 remastered versions)
Original post:
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Pro tip for anybody considering buying the Tie Fighter or X-Wing games from Steam. Don't. Buy them instead from GOG. Long story short, you get more with the GOG release and it's DRM-free to boot, while the Steam release is missing content and is obviously tied to the Steam client (well, partially, but I'll explain that later at the end).
When they were first released on GOG (Steam release came later) they came with two versions each for both games: the standard original floppy disk DOS version and a special Windows edition (released in 1998). The original floppy versions had low-res art/no 3D textures, text only, and an iMuse original MIDI soundtrack which changed dynamically during gameplay depending on what happened (subtle music when nothing's going on, battle music when enemy ships show up, etc) and the Windows editions had hi-res artwork/3D textures, speech, but a redbook-only non-dynamic soundtrack with music just lazily stolen from the movie scores which didn't change depending on what was happening in the game. This was especially an affront to Tie Fighter who, unlike X-Wing which used MIDI adaptations of the movie scores, had a fantastic entirely original soundtrack composed by Clint Bajakian. A little later on the exact same packages for each game were released on Steam.
What Steam has:
X-Wing:
-1993 (DOS floppy disk)
-1998 (Windows CD)
Tie Fighter:
-1994 (DOS floppy disk)
-1998 (Windows CD + expansion)
The missing content:
Strangely missing in both releases were X-Wing's 1994 Collectors' CD version and Tie Fighter's 1995 Collector's CD version which were DOS versions that had low res artwork (though, it was capable up to 640x480 resolution where the floppy version was tied to 320x200 only), but had speech and retained the dynamic iMusic MIDI soundtrack. So it was the best of both worlds. It didn't look QUITE as good as the Windows version, but looked better than the original floppy release and had the fantastic dynamic iMuse MIDI music system that the Windows version was sorely lacking as well as spoken speech, which meant that Tie Fighter's complete original soundtrack was intact. Fans cried out in dismay over this baffling oversight and eventually GOG added the Collectors' CD versions to their releases. Steam, however, never did. So what this means is for the same price as Steam you'll get the third missing and (arguably) best versions of each game for the same price and they're DRM-free (you don't need a client like Steam to play them).
What GOG has:
X-Wing:
-1993 (DOS floppy disk)
-1994 (DOS Collector's CD)
-1998 (Windows CD)
+Manual, reference card, strategy guide
Tie Fighter:
-1994 (DOS floppy disk)
-1995 (DOS Collector's CD)
-1998 (Windows CD + expansion)
+2x Manuals, 2x reference cards, 2x strategy guides, TIE FIGHTER Stele Chronicles
Now, technically the Steam versions can also be DRM-free if you play the DOS versions only. You can just take them out of Steam's game folder and use your own installation of DOSBox to play outside of Steam no problem since Steam just uses DOSBox to play them anyway. This isn't the case for the Windows version. But again, the GOG version is a better choice all around.
I hope this is helpful in advising people on which version to buy if you're interested in these games! They are true classics and I highly recommend the Collector's CD versions of both titles, so if you're interested please do yourself a favour and get more for your money by buying the GOG versions!