Cant see any point in having a canvas on at all in the summer as every time you want to go topless you have to rollup the sides, remove it or whatever depending on "how topless" you want to be. If you are just gonna have canvas on and roll the back up it is not worth the bother of swapping from hard to soft at all IMHO.
Why bother having hard top and a soft top, just have one on in the winter, hard or soft and remove it completely in the summer which is what I do.
A little but of rain ain't gonna kill yer. If it gets really bad just stop under a bridge in acar park or whatever until it stops.
I usually go topless early April - end Sept/mid October.