"You can't always get what you want but if you try sometimes you find you get what you need."
From what I can tell, there is no version that is going to cover all the bases - at least this is true of San Andreas:
If one is a purist, the original Xbox and PS2 versions are unaltered.
http://www.eurogamer.net/articles/digitalfoundry-2014-gta-san-andreas-on-xbox-360-is-a-mobile-port
There's debate on GTA fan sites about the extent to which the mobile version is better than the PC original in terms of elements such as draw distance and asset quality - the Xbox 360 version occupies 2GB of space vs the 4GB of the original PS2 and PC versions. Certainly the mobile release on which the 360 game is based appears to lack effects, such as the screen distortion designed to emulate heat haze, while radio station music is pared back, presumably because of licensing issues. This 360 port also suffers from glaringly obvious bugs, like audio cutting out completely during some cut-scenes.
In revisiting San Andreas, it's actually the PS2 original that comes across as the most authentic of the lot: a technological masterpiece of its time, with assets built to service what is today a relatively miniscule resolution, but at its most comfortable on the original target hardware. Curiously, despite the often crippling frame-rate, the controls just seem to feel better in some ways than the remastered version.
Scaled up to 720p and beyond, even with the enhanced assets, it's clear that the level of detail in the assets doesn't really stand up to scrutiny. The game wasn't really designed to scale in this way and it shows. At just £2.99, the Xbox 360 port is keenly priced for what it offers, but we can't help but wonder whether keeping the Xbox Originals version - at least alongside any HD remaster - may have been the preferable option.
If one is an aesthete, it could go either way:
http://www.ign.com/articles/2014/10/31/gta-san-andreas-hd-review
In the first mission, Carl Johnson silently mourns his recently deceased mother. Thanks to the upgraded textures and higher resolution (all the way up to 720p!), I could see pain in his face that wasn't apparent a decade ago.
The sights and sounds of San Andreas are the best they've ever been on consoles, but those come with the biggest caveats. While improved textures make the world more vibrant, signs more readable, and characters (slightly) less puppet-like, you'll have to deal with a surprisingly choppier framerate. It's usually manageable, only really manifesting when the camera is moved around quickly or too many cars are on the road, but it can become distracting when it plays out during action sequences. There's also that issue of audio suddenly muting during cutscenes roughly 10 percent of the time, which is stranger than the graphical issues since San Andreas' epic soundtrack and star-studded vocal performances haven't been altered.
If trophies are important to you, then the later version will be preferable:
http://www.ign.com/articles/2014/10/31/gta-san-andreas-hd-review
It's worth noting that this is the first Xbox version of San Andreas to incorporate Achievements, so if notifications and meta-points happen to give you additional motivation, you might find additional reason to explore, experiment, and complete the campaign.