You can't answer this because it depends on the total equipment available. I've done gigs with my amp and cab in the van, using the decent monitors and a DI. The gig I played last week provided me a HUGE Ampeg system, that was running really low, because it was too far away from me, so again, my monitor did the work. I rather like Carlsbro equipment - always have, even though it's not thought that highly of by many, and I have a 2 x 10" 60W combo and a bigger head and 4 x 10" cab. Very often my choice is made once we see the space. If the supplied PA is a bit lacking at the bottom, I'll use the bigger cab and turn up, but if the system has good bottom end, then the small combo is fine. I like the sound of 10" speakers, so when the combo started to be a bit compromised, I just bought the same, but bigger. I'm happy with a DI or a mic into the PA.
In your case, what is going into the PA - everything? So the PA does the work, and you get a quiet stage, or do you want/need to be as loud as the drummer on stage? If monitors are rubbish, then you need more oomph. So many variables that any kind of rule is a bit pointless - BUT - having access to a 300W system will be more versatile than 60W. Nothing says you have to run the big system loud!