It's more of a "Buick vs. Chevy" issue, actually, since what we're talking about is two "feature points" on the same "design model."
So. My little brother was hunting for a concealed carry handgun last spring, and, after discussing the options in real depth, we decided that one of these 3.5"-barreled single-stack S&Ws was the way that he should go. He
wound up with a 3913 (actually, the desirable 3913 NL model, so fancied by gunwriters and other cognoscenti
http://www.thegunzone.com/3913nl.html), but I advised him that he'd be well-served with a 908 if that's what he found.
The differences between the two are really minuscule, much less dramatic than those between the high-line Kahrs and the budget Kahrs, for instance. Functionally, I don't think there'd really be any difference at all between the 908 and the 3913 (whereas there's a demonstrable velocity difference between the polygonally "rifled" barrel of the Kahr P9 and the conventionally rifled barrel of the cheaper CW9). I don't see the plastic parts on the S&W "value series" as ever making any practical difference in most users' lives. And the slight aesthetic differences? Hey, look at how many people carry
Glocks - I'm not sure that aesthetics are at the forefront of handgun purchaser concerns.
Good luck with whichever you purchase, but I truly think you're fine whether you buy the "Chevy" model 908 or the slightly fancier "Buick" model 3913 - they're the same in every way that counts.