the mandatory installation is because the PS3's bluray drive is incredibly slow which would make load times ridiculous. There's not a huge difference in the Xbox 360's DVD drive and it's hard drive, so installing to the HD would be a very very slight difference (The 360 has a very fast DVD drive and the 360's HDD isn't very fast). It wont help with the frame rate, it will only effect load times.
Pop-in on the 360 version COULD be affected by load times, however, since it has to stream from the DVD, which will be slightly slower than the streaming from the PS3's hard drive.
I own a 360 and PS3. I'm getting it for the 360, as I do with all games, because all my friends play on the 360, and the online service is better. It doesn't matter to me which one looks slightly better or runs slightly better. Both versions are killer apps and the best game of the year with (probably) no competition whatsoever. I could live with either for a LONG time.