The Studio20s are a better speaker than the B25s...and I'm a big PSB fan! There I said it!!
You're in a tough spot. If I knew I could dispose of the unit (in less than 5 years) that you are putting the speakers into, I would go for the Studios. But I still would not buy the PSBs either if I was stuck with a wall unit. I would opt for something cheaper because in my mind, sticking speakers into a cabinet severly restricts a speakers ability to image properly. I would be afraid that the sound field would sound boxy and just plain bad.