I'd say buy one with a 256gig drive.
I have a UX31E (the TN-panel version) and it's a great machine.
But HDD (and SSD) tend to fill up rather quickly, and I prefer to do maintenance once every 6 months instead of every couple of weeks or so.
Then again, I work with a lot of mediafiles, iso's, RAW files and RED footage, so my usage pattern may be atypical.
I7-3770@4ghz, Asus P8H77-M, NVIDIA 780 GTX, 16 gig DDR3-800 Kingston, 1x Samsung 840 EVO 512gb SSD, 2x 3000GB 7200rpm SATA WD Black, Lite-on Blu-ray 16x DVD+-RW, X-fi Titanium HD, Corsair 550D Obsidian.
Asus UX31E with i5 and 256gb SSD.