I've seen both multiple times (my daughter is obsessed with "Altar Boyz") and have used Forbidden Broadway as kind of a stopgap when nothing else strikes my fancy.
It's clever, silly and dependably funny and for $30-35 (at TKTS), worth a look.
See Forbidden Broadway if you're familiar with most of the shows it's parodying. If you're not, you won't really get the jokes so you'd be better off seeing Altar Boyz.
I'm only saying that because you've seen Altar Boyz twice already. If it was a straight choice, I'd pick Altar Boyz.