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.
Altar Boyz! The songs in Forbidden Broadway are very hit or miss.
"In theater, the process of it is the experience. Everyone goes through the process, and everyone has the experience together. It doesn't last - only in people's memories and in their hearts. That's the beauty and sadness of it. But that's life - beauty and the sadness. And that is why theater is life." - Sherie Rene Scott
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.