What LP said. I have a 26" Matrix. + If you're tall - inseam above 33" you may need the large size frame.
There are a couple of Jack users on here and they seem pleased with them, and their versatility.

See this thread about Owlmeister's quest for a bike which may have some relevance.

I almost went for the Jack, but figured I'd get more offroad use from the Matrix, so that was the beast for me. It's working out very well.

If you think you'll need more gears though, it's easier to get them at this stage..