I think its pretty obvious given that he had high cap 9mm magazines and the power difference between the two, that the 9mm glock was more lethal. IIRC he used both at once, so it would be hard to tell which one killed who. I don't remember any reports breaking the deaths down by the caliber.
Regarding it being an odd choice of backup arm, I think he didn't have much choice. It was apparent by the profile of the shooter that he didn't have much about life figured out, and that he wasn't very successful. In turn, he probably did not have much money or a good job. So then he did not have many guns, and likely only owned those two. If he had several to pick from, and chose that P22, which aside from cal is a lower quality gun, then you would have a point. However, there is no info to support that he had other choices. He could have saved money towards a better gun than the Walther, but with people like him, the right place and time (to him) is more important than having perfect weapons. He knew he would catch everyone by surprise, he knew it was a gun free zone, etc and so he did not need 2 great guns, or an AK or any other better group of weapons. Unfortunately, he knew he would kill innocent people anyways.
It was a total tragedy and the idea that gun free zones make everyone safe makes me sick.
