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

Good and bad to both... Here are just a couple of items.

Pros:
Merc has better resale value, especially in the midwest and northeast.
Merc has more certified techs and service centers
Merc builds some pretty awesome stuff
Volvo builds a good quality product that may be slightly better than Merc
Merc is phantom black

Cons:
Merc is the "big dog on the block" and is not very responsive to customer problems whereas Volvo seems to have a better rep in that department.
Merc does not take care of service for warantty work very well. In my case they have not paid the going labor rates to my service center and that makes me responsible for what is charged over what Merc will pay.
Volvo is steel gray

you should ask CSpray from the Board. He has a few Volvos. I believe the Volvo has an integrated hydraulic steering....That's a BIG PRO!!

Good points Viper340 - if i may add;

Volvo / Pro's
has intergrated steering builtin
dual props - very efficient
TWO YEAR FACTORY WARRANTY

Con's
availability of parts and or dealers
availability of aftermarket high performance enhancements

Yep - I have three 600DPX's - 2 in a 38 Formula and one in a 28" BatBoat.

Pro:
"Free" hydraulic steering. ($$$)
Great handling, (especially in a single-engine boat!), VERY stable.
More efficient at cruising speeds.
Awesome acceleration.
10-15% stronger than a Bravo. BIG propshafts.
Great shifting - electronic shift interrupter.
Engine's still a Chevy - any hop-up parts will work.

Con:
Depending on your area, service/parts can be harder to find.
No aftermarket props (but the VP ones are very good).
Currently no engines more powerful than the 8.1 (420 hp)
Sometimes a tiny bit slower than a Bravo.
Non-standard transom cutout (as if it matters.)

I'd do it again.