They will both perform roughly the same and both are well known and regarded brands when it comes to netbooks.
The hard drive size only effects the amount of stuff you can store on the netbook, and 160GB is enough for most peoples needs (as a rough idea, 160GB would store about 32000 music tracks as MP3, 250GB would store about 51000)
However, unless things have changed lately, I expect there are other differences, the licensing model for XP on netbooks states they can't have more than a 160GB drive in them, whereas the license for windows7 allows a larger drive, so I'm guessing they also come with a different versions of windows? If that is the case I'd go for the asus with windows7 (but that's personal perference, you might prefer windowXP on it)