when i was playing in my old hardcore band we often played bars and places that had no PA. I 15, 25 or even 30 watt would have struggled to cut it at times. You need the headroom in a high gain amp, otherwise you end up with a massively distorted preamp signal going into a massively distorting power stage. It doesn't sound good. But if you're packed in next to a drummer and bassist and only the vocals are going through a PA, then you'd be surprised at how loud you have to be to get heard. But, use your experience to figure out what tools you need.
As for tha 6505 vs the 5150, the circuit is the same and they contain pretty much the same parts. The 6505's however dont have the power valve pcb riveted to the chassis, so it's much less of an issue to repair broken screen grid resistors in a 6505. You just un screw the pcb in the 6505, but you need to drill the rivets out in an old 5150. Those resistors are mounted under the pcb. The other really noticeable thing is at some point peavey swapped the ribbon connectors for push on connectors with individual heavier gauge wires. This probably helps stop the connectors burning out and in some cases destroying huge tracks of PCB. So even though the older 5150 is cooler, I'd say the newer ones should be slightly more reliable and easier to work on.
Still, a second hand 5150/6505 seems hard to find for less than £550 these days.