Get the hardtail. I actually have it and the reason i got it over the fx is because its cheaper and the fx has a crappy in house giant rear suspension. But if you really want the fx it come with a better fork and i think a better rear derailleur.
Well normally I would say go for the lightest as the up-hills in cross country can be grueling if you are on reds and blacks. The weight difference here is only about 1 kg which is not huge. As for size, you will probably be a "small adult" but go try some at a good bike shop and get some advice. At your age, if it is going to have to last a year or two, I would go for a very slightly large bike. If you buy a perfect fit you will have out-grown it buy Christmas.
(Have you checked out Scott)
Always start on the hardtail if you are new.
As a beginner, you should focus on skills first. This includes basic bunnyhopping and shifting weight on the bike.