the question has been....low mass/high velocity or high mass/low velocity. After a lot of reading I'm now in the "use the highest mass that you can keep above 1000 fps" camp. This achieves good penetration and if you use a modern SD designed bullet (like an FTX or Gold Dot) it will expand as well because these are designed to expand at achievable handgun velocities. (typically above 900 fps). This always comes with the caveat that you have to be able to hit a vital area. Also, expect to fire more than one shot. I think the statistic is that most quick stops are achieved with 2.24 shots. So firing two and one quarter shots would be best policy. Of course if you can carry and control a more powerful gun that can shoot a higher mass bullet at a higher velocity, go for it and render these theories moot.