If you've never played either, the sensible choice is SH3, not because it is any better or worse (they essentially involve the same thing), but simply because it is cheaper, and why spend 30 quid to find out if you like something when you can spend 10 quid to do the same?
I have them both, like them both and play them both, but SH3 will run on a lesser machine and has more free mods available for it, whereas SH4 is prettier, has quite a few mods, and has a mission disk coming out for it next year, but requires a better computer with better graphics capabilities to enjoy it with all the detail turned up.
If computing power, money and personal theatre of operations preference are not issues, then get both!
Chock