>I'm budgeting for a 15" light bucket. < Holy light-bucket, batman! They only thing I can suggest (and you've probably already done it) is to check Sky & Telescope for reviews on such products. Lastly, talk to someone who has used a scope that big before; one of the lessons I've learned about amateur astronomy is that it doesn't matter how big your scope is if it just sitts in the closet all the time. If setting up/breaking down and transporting a 15" scope is a pain, it is unlikely to ever get used..... unless you are a fanatic, which, since you're contemplating a 15" scope, you probably are.
Best of luck. Personally, I'd find the 4" refractor a great deal more user friendly.... but that's a personal opinion.
I love my refractor, and have spent many hours lovingly caressing the planets ... but it's just not enough anymore. I'll keep it, though.