the xps is smallee, new!! and better for storage
the alienware has a stronger graphics card, and theoretically more cpu upgradeability. However in my experience of Intel, even with the same cpu socket, the chipset normally doesn't support more then 12 months of cpu releases and i wouldn't upgrade for a few years
That depends on what you are looking to pay for the systems. You would probably be able to do so much better to build your own or if you know someone to help you build one. You would have a lot more upgradeability if you stay away from proprietary machines (especially DELL which owns Alienware). If you want to go with prebuilt, then I suggest something from iBuyPower or Digital Storm. Others on the forum might have some better insight on to which custom build retailer to use. Never have used them personally.
I understand. I used to build my own systems, however I stopped due to lack of time and annoyance zeroing in a fault in a few cases. i am in the UK and did price up both an own build and one of the custom PC companies. the custom PC companies were more expensive and the own build was cheaper, but not that much for an i7 machine