RE:Double tapers verses specialized Spey lines
Funny you should ask - I too enjoy the longer belly lines for summer/dry line work but don't enjoy casting sinktips with them.
The best compromise I've found is to have the longer belly lines (TT Spey, SA Spey) if you're limited to one line. This is long enuf to have good mending capabilities for dryline work and short enought to cast comfortably in winter with light to moderate tips.
The primary drawback to these long(er) belly lines is with the Really Heavy Heads sometimes needed in winter/spring (Deep Water Express) - these are best fished with the Windcutter/30' floating head/launcher lines where the length of line being cast is short enough that the additional weight doesn't compromise the castability of the line and are heavy enough (they're usually made out of 12-13wt floating line) that the sinktips won't submerge them. These lines aren't really cast - it's more like lobbing a really loooong slinky- but their performance is unmatched when it comes to fishing the heavy tips that really get the fly DOWN FAST.
These line's aren't the pleasure to cast that longer belly lines are, but I'll sure keep them around because when you need them you Really Need Them.
My $.02 worth,