Forget about both of them for the time being. Buy next year. Because we are going to have pretty serious competition.
imho, Intel doesn't have the right platform (chip) for their so-called "Ultrabooks"....sure, they use special "ultra low voltage" binned Sandy Bridge processors which are good processor wise. At the end of the day, Intel HD graphics suck and you can't game on them.
if you are going for a portable, your requirement is mostly this: 1) Battery life 2) Check out mail and carry out basic tasks 3) Quick boot through a SSD 4) Watch videos without draining battery 5) Ocassional gaming
AMD with their Fusion processors on the other hands, give you a weaker processor (wait for next iteration named Trinity) than an Intel ULV but much much better graphics built-in. Fusion = CPU and a proper GPU on the same chip. Laptops with discrete graphic cards are battery hogs.
Take this for graphic intensive benchmark for example.
AMD A8-3500M (Llano) uses 50% power compared to i5-2520M. This means it will last ~two times more on battery. And obviously with AMD's superior graphics, A8-3500M will run this graphic intensive benchmark ~2 times faster than i5-2520M.
*i.imgur.com/Dwmfz.png (power consumption under graming)
*i.imgur.com/sUM4w.png
*i.imgur.com/hUQlc.png (twice as fast in gaming)
Wait for AMD to come up with "Trinity" in Summer. That is going to get better on every front. - I expect Intel to be kicked out of the low-end laptop and Ultraportable notebooks segment. AMD E-350 has already knocked out Intel Atom from the netbook segment. Intel will only rule high-end "Laptops" in coming years. Not portables.
Note: I know i5-2520M is NOT an Ultrabook processor and processor wise it will rape A8-3500M - but a strong processor is never there in an Ultraportable. I'm only trying to give you a proper idea that this market is going to see "uthal-puthal" soon. So, it is not wise to get one "Ultrabook" at the moment.