I wouldn't spend a cent. I'd use what you have and insist that the band play down to your volume capabilities. If not, take your amp and throw it at the guitarist for being a douche.
This, while no being the most friendly advice , does make a point.
I'll take it a step further, does the guitarist want help carrying the PA? And extra cut?
But then to be a realist what kind of PA is it? If it's one of those Fender Porta PAs it's not going to much for you anyhow.
Keeping the volume in line with the gear that everyone has just makes sense. All it takes is one Marshal stack getting his tone at 126 dB to wreck that.
Sit on top of your amp if need be. Do not go blowing up your speakers just to try to keep with a power crazed watt head.
If you can train them that's good. Show them articles about Jimmy Page getting his Les Paul through a Marshall tone using a Tele and a Supro. And let them know that Clapton's amp of choice is a Champ.
And if you can learn them to think that a band isn't a race or a competition to see who's the loudest you would be doing a public service.
Renting another amp would make next to no difference. Unless you went large expensive and heavy.
Then you'd be better off hitting the used market to upgrade.
But the fact is that amp is loud enough for anyone with any connection to reality. If they push you to upgrade your amp push them to upgrade the PA.