I've run most of both ICS ROM's(Except the latest ICSSGS) from the start.
To me they are both very similar in performance, I feel THS is a bit better.
I'm not a HUGE fan of pure ASOP Rom's, I like having a little bit of customization, which I get with THS's ROM. I haven't been completely been following ICSSGS's thread since THS came out with Build 14. I was VERY happy with it, B14 was filling in all of the little quirks that I was not too happy with and didn't feel the need to get into ICSSGS's V3. Some folks have been having SOD issues with THS B14/15, I have not had any myself until I started messing with kernels.
Now I don't want to say that I was at all unhappy with ICSSGS's ROM, it's a really great ROM, but with my device(as we all know, each can be a bit different). I feel THS was doing it a little bit better for me.
Hope this info helps some