I've been an Ampeg user for years now, though I've used Fender, Peavey, Hartke, and others. What I've found is this:
More power is better for most folks.
More power generally means more headroom.
Better to divide your signal, if you can, amongst different speaker sizes so you can get a balanced sound.
If you can't divide your signal, decide which tone/sound appeals to you most and go with that.
Like Keith, I remember the days of heavy amps and cabinets. I still live those days now due to my choice of Ampegs. However, the tone is perfect for me. I run my SVT IV through an SF-2, and the outputs go to two Ampeg BXT series cabinets (1X15 and 4X10). I run the Low Pass to the 15, and the high pass to the 4X10, which I run full range. It works for me. With 490 watts per side (the SVT IV has a built in crossover) at 4 ohms, it's plenty loud, too. Keith has played through it, so he can vouch for the tone/power. We couldn't turn it up too loud when he visited, though!
Bottom line is get what works for you. I've been playing for 28 years, and I'm still learning about tone, rigs, power, etc. If you like 15s, that's all that really matters. After all, it's your money and your choice.
My two cents,
Alan