Im building a new 25'advantage citation with a merk 8100 or
volvo hp500 dpx. pros and cons?

I'd stay with Merc. It is very easy to find parts and service for the mercs. The after market stuff seems to be centered around merc and their drives also. Here in Atlanta volvo stuff is hard to get and service is available at only a few places. There's very little you can do about props too.

Just my 2 cent....But remember, you get what you pay for.