It mostly depends on the type of riding you want to do, and also what will fit (I.e, how tall are you?). Your budget is important as well. If you want to race Xterra, you should focus on a XC race bike. Trail bikes handle more difficult terrain. CX bikes are for cross or gravel rides and not mtb (unless your trails are flat and smooth, or you're Sven Nys)

I this your one bike beyond the Tri bike? Do you own a road bike?

The other key question is, how technical will the riding for you do off the road?

If your off-road riding is going to be non-technical ( gravel/dirt roads, grass, easier trails), and you currently down't own a road bike, then a CX or one of the new gravel road bikes is a great choice. This one bike will give you extraordinary versatility in terms of where you can ride and the type of riding that you can do. With a simple change in wheels and/or tires you can go from the trail to riding a gran fondo on the road!

I wrote a bit of a story on the gravel road riding phenomenon, with a bike review thrown in on the Raleigh Tamland, here - http://stevefleck.blogspot.ca/...road-less-taken.html

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Steve Fleck @stevefleck | Blog

Thanks Fleck,

I do have a roadie that I use from time to time, mainly ride my TT bike. I have ride a 51cm cervelo TT and i want to say 54 maybe 53cm Giant TCR roadie. As for riding goes, being in the northeast i can get as technical or nontechnical, easy-hard terrain. I want to try it but have no idea where to start. sizing looks different dont know much about whats good and whats not. there are some single track trails around me with rollers and so on.