I've posted this on another thread, but it looks like Dell is trying to make real laptops. The newest XPS ultrabooks have received rave reviews, and some of them even ship with Linux preinstalled [1]. That said, my personal laptop is a Lenovo T440s, because IMHO Lenovo still wins on upgradeability. I was able to buy a base model, and in short order had the RAM upgraded to 12GB (the max this model supports) and swapped out the hard drive with a SSD from Amazon. I'm not sure how feasible that is with any of the other laptops being discussed here.
I just bought an X250 after searching around for an alternative. The XPS 13 looked pretty good and I was about to buy it ... but no physical nipplepoint buttons. Maybe next time I'll be willing to risk it. (Yeah, the X250 looks like it has plenty of downgrades from the X201, but it's better than a T440p, and at least it's a known quantity whereas Dell has an even worse rep...)
I'm typing this on an X230, with 16GiB RAM. I busted the screen recently and considered an X250... it's such a shame that "ultrabook" means "one DIMM slot" these days, i.e. they max out at 8GiB RAM - same as I was using in '2009 on my old X61s!
Luckily replacement screen parts are quite affordable these days. And before anyone complains, I do use all this memory, currently dom0 has 4GB free.
Good news! There are 16GB SODIMMs from "I'M Intelligent Memory". You'll have to look around to figure out what model exactly, but people have put them in new X series and they work.
http://www.dell.com/us/business/p/xps-13-linux/pd