Thanks for this comparison. It's an interesting one.
The C64 version looks good, it's fast and all, but did you notice it's one flaw? The scrolling is too slow. When you walk without standing you reach the border of the screen and the camera slowly follows. This is not in the arcade version, and it can be deadly. Really an uneccessary design flaw that screws up an otherwise good version.
Also interesting: The SMS and GG versions are completely different from each other. This is rare, as usually developers just did the same game on both and saved money due to the almost identical hardware.
Furthermore, of all versions the SMS version is the most different from the arcade. It lacks scrolling but has flip screen for some odd reason (even the Speccy has scrolling!), and it has completely different aliens. They tried to make them look more like the movie Aliens here.
The ST version looks very good, too bad no Mega Drive port was ever made.
Also, a common problem Sega had in those days: the game was on NES. Sega was really stupid to grant licenses to 3rd parties that allowed them to port their games to competing platforms. No wonder they failed so hard in the US. Afterburner, Shinobi, Alien Syndrome, Altered Beast, Fantasy Zone, Space Harrier all were Sega hits, and they all appeared on NES. I presume Sega gave those licenses before they released the SMS, but even then some planning that chances are good you will want to sell your console in the west would have made sense.
To a lesser extent it probably hurt European SMS sales as well; while the SMS did well, I imagine it would have done even better if people had not been able to play most of the games on every homecomputer on the ace of the earth as well.
Nintendo sold their systems because they never let go of their proberties after the NES was out, you could not get their games anywhere but on Nintendo systems (save for the short CD-i intermezzo).
BTW, here are some missing versions:
X68000 (superior to the arcade!)http://www.youtube.com/watch?v=z8dAGvEb290
And only a sceenshot, beause I did not find a video:
A schameful CPC version...