The 908S is a low cost version of the 3913 I think. It does not offer an ambidextrous decocker/safety and also lacks the accessory rail of the TSW3913. Otherwise the models are almost identical. Smith and Wesson third generation semi-auto pistols are , in my opinion, too often overlooked in favor of the "sexier" Glock and Sig models. The S&W models are quite reliable and offer good, if not exceptional accuracy. I carry a 6906, the discontinued, double-stack, 12 shot version of this compact pistol, and I have never had a problem with it. The 3913 and the Sig 239 are very similar in size. I suggest you select the one that feels best in your hand. Also consider holster options if you intend to carry. FS is very accommodating to set you up with pistol/holster package that works for you. In any event, you can not go wrong with either pistol, just stay safe, and practice a lot. Hope this helps.
Regards,
Chris