Hey guys! I have been riding road for more than a year now, and I have been having a craving to get a MTB for the last month or two. Unfortunately I am still collecting money, so I will not be getting a bike for at least 3-4 more months. But I just want to get an idea of what I should be looking for.

I think that most of the trials in the area I live in(SoCal, Thousand Oaks/Simi Valley area) are fire roads and gravel trails(correct me if I am wrong), so I think I shouldbe looking at Trail or XCountry bikes.
I am 5'2 and ride a 50cm CAAD9 road bike, what size MTB should I be looking at?

My price range is around $800, so i am guessing I should be looking at low level bikes or used, I'm just not sure what to look for on craigslist or ebay. I would like to get a new bike, but at this price point, I don't know if that is a smart idea.

I talked to a couple of guys I know who mountain bike, and they recommended Giant for the amount of money I can spend.

I don't know if their bikes are any good.
Two I have looked at online: Giant Revel 0
http://www.giant-bicycles.com/en-us/...el/9042/48917/

I also saw the Giant Yukon FX
http://www.giant-bicycles.com/en-us/...fx/9040/48913/

It is a bit more expensive, but I might be able to find one used or a previous year model.

So do you guys have any suggestions? And tips? Thanks!

PS. I don't really know much about MTB components or anything, I am a roadie at heart.