Honestly, both games are great; however, keep in mind your paying $15 bucks for Super Turbo (a game thats been out for over 15+ years) with minor touchups and pretty sprites. SF4 on the other hand; grant it that its a SF2 homage, but the game has alot more elements and mechanics than SF2HD. To each his own thou. :p
SF4 is cool, but I don't know I'm just not a huge fan of it. It's really slow paced and has some features I don't really like at all(I personally HATE the saving attack system)
HDRemix is a rebalanced re-tooled version of what is probably the most influential 2D fighter. Just with re balances and smart control changes. Plus the online is amazing
So I'm kinda leaning towards HDRemix. SF4 is alot of fun but there are aspects I really hate about it.