Of the 2 guns that you mention, I would get the SP101 for several reasons.
First, you won't have to start buying another type of ammo right away. On that point, 38+P ammo is usually more than enough for defensive purposes. Magnum loads in small guns are much harder to control which makes follow-up shots much slower, whether its on the same bad guy or on a second bad guy. Accuracy also suffers if you haven't had lots of practice with magnum loads and who can afford that these days?
Second, the SP101 is quite a bit easier to hide than the Glock. Even if you get it with the 3 inch barrel, an option that I strongly recommend. You lose almost nothing in carry convenience and the extra sight radius and muzzle weight make shooting quite a bit more comfortable and more accurate. You'll also get a bit more velocity from whatever load you put into it. In 29 years as a cop, I've almost never gone on duty without a backup gun. Usually it was one of my 2 inch S&W Chief's Specials. I acquired a 3 inch Chief's Special a number of years ago and none of the 2 inchers have ever come out of the safe since then unless I needed to carry in an ankle holster and that was pretty rare. (Some of the guys who are taller than me have no problem carrying a 3 inch revolver in an ankle holster so you may be able to do so also if that is one of your anticipated needs.)
Third, the SP101 is cheaper than the Glock and that leaves you more money for practice ammo! A good training class wouldn't hurt either!
Good luck with whatever you choose!