In my opinion, its better to have to many, then not enough
Buy a 300 watt amp, you'll never need 300watts at a jam, run it at a quarter so you won't be pushing it to its limit. You'll also have that lil bit more oomph for larger gigs. If u plump for 50-150 watts, theres a chance that you'll be running it flat-out all the time, not ideal!
I have a 120watt practice, its cool for acoustic jams, but once you add drums, the overall volume increases considerably.
Headroom is the order of the day for me
Si