Depends what you're after.
If you want the best portability possible, but at the expense of heavier power and graphics, then Zenbook all the way. If you need more power, a solid graphics card and more HDD space - and don't care you're carrying a couple of kilo+15" notebook, get the N53. Check the relative display resolutions too. The N53 is an older, 2010 model - the N55 was launched later last year (around the same time as the Zenbook) - even if they both can be fitted with Sandybridge.
