Hello,
Are you in the PDX area? I ride my mountain bike up in Forest Park every other weekend or so (unfortunately i've morphed into something of a roadie in the past few years) and lately i've been taking my CX bike and riding the same trails i've been riding my MTB on. Yahoo!
I'd say go with the CX bike. If you put some skinny road tires on it-- presto! A decent road machine. Some knobbies? Ride whatever you dare!
One thing that i've noticed about riding my CX bike: It's very "retro" and reminds me greatly of my early experiences riding off-road. No suspension and the philosophy "If you ain't hinking, you ain't mountain biking" ruled. Now it seems that the technological development of mountain bikes and the mindset of their riders is to "ride everything". Suspension, fat fat fat knobby tires, and low gearing go a long way towards this.
I've had to dismount and run up obstacles, run down super steep and technical descents; dismount and "suitcase" the bike over potholes and drainage trenches.
This return to the "roots" of mountain biking is super fun-- riding my CX bike off-road has revealed a little of the lost joy of cycling for me.
I have a couple of recommendations for you, as you purchase your CX bike:
Buy good brake pads right away. Kool-Stop's dual compound "Mountain Pads" made a world of difference in the braking performance of my bike. The pads are black in front and "salmon" colored in the back.
I found the traditional road-style drop bar (even the ones, such as Bontrager's Race CX bar, or Richey's Bio-Max that are supposed to be designed for CX) are pretty terrible for serious off-road control and comfort. Get On-One's "Midge" handlebar. This is a TERRIFIC handlebar-- so comfortable and very secure feeling. It's style is sometimes referred to as a "dirt-drop" style.
www.on-one.co.uk
They have a US distributor and thus their bars can be found on many on-line resellers.
www.webcyclery.com
I picked mine up at Veloshop in downtown PDX.
Enjoy!