Also i'm lo
Someone here a few pages back was wondering about the compact X51 with a graphics card upgrade. I can't really recommend it as the X51 comes with a tiny 330 watt power supply that may or may not be able to handle the GTX 980. I'd look into either of the two other desktop PCs.
That said.. the XPS 8700 and Alienware Area-51 both vary quite a lot in price, assuming USD the former begins at $600 and the latter begins at $1700. Hard to compare the two when we don't know how much you're spending or what PC specs we're looking at.
I and most of the others in this thread also don't really recommend the GTX 980 as a purchase on its own because the extra $200 over the GTX 970 only amounts to a small performance increase, some GTX 970 models can even match the GTX 980 with some tweaking.