Good point, HaLo... I didn't notice that either. I don't know if backspacing is the same as S13s, but I know that you can fit at least a 1/2" wider wheel/tire on an S14 than you can on S13... my guess is that backspacing is similar, though... but with even more room back there, a +40 wheel will just look worse (more sunken in... although some people like that look...)
If you want to be safe: go with 215.
If you're willing to take a little bit of a risk: go 225.
If you're buying a **** tire, go 225. If you're buying a nice sticky tire, go 215 (you reallllly wouldn't want to rub on a nice expensive sticky tire, whereas cheapos are disposable... to an extent.)