I have a Mark V:25. It's a great amp. Fantastic cleans, fantastic Crunch, fantastic high gain. Will said the 35 gives way more volume. That may or may not be true, I haven't tried the 35. But I can say the 25 gets very loud even through the 1X12 cab. I haven't run mine through my 2x12. As much as I love its tone, I'm considering selling it.
The things I don't like:
1) The Crunch mode is on channel 1 so I can't switch quickly between clean and Crunch. Crunch is my favorite mode on the amp.
2) I wish it was 3 channels (goes with number 1)
3) I wish it was organically MIDI capable
Those things really aren't true showstoppers, but I'm planning on a stereo rig with the TC-50 and a Peavey Invective. so I consider selling it but every time I look at it I think about how awesome that little amp is and wonder if I should just keep it.
I've owned both the 25 and 35, and I definitely disagree that the 35 is a lot louder. Maybe a little more clean room. Certainly not worth the increase in cost/weight/size. I'd much rather get the 25w, and a Fryette Power Station if I need more juice.