Trying to decide pre-built or build.

Hello. First time here.

Normally I build my own machine. I have since about 2005. This time, due to space limitations I am wondering if it makes more sense to buy a pre-built. I would like something smaller that I just hook up to use for occasional games. I really use my Surface Pro for everything I do, but it's not exactly a gaming machine. I'm not as into tinkering and overclocking as I used to be, and I've never built anything with a case that limits the HW options.

I am liking the size of the Alienware X51, but at $1600, is there an option to build one the same size or similar while keeping as powerful or better? What I like about the X51 is that it looks easily hidden under a desk and something I can just grab and connect up to with little effort when I feel the urge to game. I don't like console gaming.

For a reference, most of my gaming is Final Fantast ARR, and GTA V.

Thanks.