I like the .45 and the 9mm very much. The 9mm is much cheaper to shoot Winchester Value packs are around $10 for 100 rounds, and Blazer can be found for around $5.00 per 50.

I like the recoil of the .45 but some don't. I would try and shoot both and see which one feels the best to you.

If you want to see some data on defensive loads, look here http://www.ammolab.com There is plenty of good 9mm and .45acp defesive ammo out there.