Stainless will not be that much of a weight difference to be of any real significance. I also hear the cracking issue come up all the time, and yet look at your bow rails on most all boats..... better yet watch them flex and move with very little if any cracking issues.
I have seen numerous T-tops done in stainless that have held up fine, and the same goes for Aluminum..... then again I have seen some cracking issues on both aluminum (quite a few actually) as well as some stainless, and many times it's either from not building the Top correctly or some guy that like to play baywatch patrol and pound his boat extra hard by jumping waves.
I'm not totally against aluminum, but if I had the choice between the two without money or anything else being a factor, I know what I would be choosing.
The unknown fisherman