I have the Ruger SP101 2 1/4" barrel and this ammo for defense:
www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=378809
Never had to shoot anyone with it - yet. I am sure it would do the job.
As far as using semi-jacketed ammo, IMO it is better to use ammo that is specifically for defense for defense. Semi-jacketed ammo would have to suffice if better ammo is not available - though would you want bet your life on the semi-jacketed ammo so you can save money (not saying your cheap or anything - just some people think the defense stuff is not worth the higher cost).
I would suggest getting a few boxes of defense ammo (Federal, Winchester, Speer, or others) and see how you shoot with them. See how you like the recoil, see how they shoot and where they hit over various distances (7feet to 21feet), and if possible recover some fired bullets and see how they expanded (that is if you are shooting into something that will make them expand rather than shooting at a metal backstop - perhaps with your own outside test firing range somewhere in PA where you can setup and shoot phone books or something that will cause expansion where the bullets can be recovered). This test shooting of the ammo would be somewhat expensive at approximately $16 - $25 for 20 or 25 rounds of each of the ammo you choose, but will be a one time testing and how else will you determine whether you like the recoil and accuracy that the ammo will have it you don't shoot it.
By the way, you might want to check out Gemini Customs:
www.geminicustoms.com/RugerSP101prices.htm
They do awesome work on SP101's and you might want to think about what you might want done - if anything - to yours. I can't wait to send mine to him to get it modified (I don't want the porting or the bobbed hammer though).
Luke
