There are some crazy discounts every so often (I got a 35% off on my X250 order a couple weeks ago; I spent $1500 w/ shipping+tax for the i7-5600U CPU, 512GB SSD, 12.5"FHD screen, w/ a 6-cell battery). It maxes out w/ an 8GB DIMM, but 16GB DIMMs are starting to show up and will be compatible (albeit currently quite expensive).
The X250 is slightly bulkier than the MBA11/XPS13, but one nice thing is that it comes w/ a built-in 3-cell battery and an additional hot-swappable 3 or 6-cell battery. Real world battery life is reported to be 6h+ with the 3+3 combo, and 10h+ with the 3+6. Lenovo claims 20h in Windows. I'm interested in running some tests but I'm confident it should last at least a work day.
Here are some notes I've gathered, both comparing various ultrabook options (I'm replacing my MBA11 after multiple generations of disappointments) as well as lots of X250 specific config notes: https://randomfoo.hackpad.com/Lenovo-X250-Linux-CVpUMLjBDZh