I second cutting the 15.5 into the keggle. If you want to do 90 or 120 min boils you'll need at least 7 gal to get 5.5 gal into the fermenter. Add in that you can boil 10 gal batches in it and I think that it's the clear winner. As far as fermenting, you can make a 10 gal batch and split half into each of your fermenters. Or, for ~$30 you can go buy two 7.5 gal buckets at your LHBS and split into those to ferment. On the other hand, if you cut the 7.5 gal into your keggle and later on decide that you want to do longer boils or 10 gal batches you'll need to go spend $30 or more on a new 15.5 gal plus new fitting for it, cutoff wheel, etc. That or buck up $150+ for a 15 gal pot.