I ended up going with the k701s for a couple of reasons.
Soundstage / imaging - Soundwise I preferred them over the HD 595s. The imaging/soundstage on the k701s is very wide, which I like as it helps me pinpoint where that gunfire is coming from. Although not as warm sounding as the Senns, the bass is definitely present, and I expect the bass will continue to present itself as these break-in. I also like the openness of the mids and highs on the k701s. For me, this is important in gaming, probably more so than bass response as the higher frequencies tend to trigger me for directional cues better than the lows. I would be happy with the sound of the Senns as well, but between the two, I like the k701s. (Of course, they cost $100 more too.)
Comfort - the other reason I like the k701s is they were more comfortable to me, particularly upon using them for extended periods (3-4+ hours straight). Wearing the HD595s for a long time caused me some minor discomfort on my ears, in the form of soreness on the outside of my ears. I think this is probably something that's particular to me, though my wife had a similar reaction. The k701s have a larger, rounder earcup that must just fit around my head/ears better. (I imagine the k601s would feel similarly comfortable, but I don't know for sure.) I don't want to leave the impression that the HD595s are uncomfortable, and to be honest they might have gotten more comfortable if I had kept them longer and tried to break them in more. I read many posts about people saying the Senns are plenty comfortable.