I guess it mainly comes down to whether you think the Acer is "too small" or the Wind "too big", however there are a couple of additional small differences:
The Acer keyboard has the Fn key on the inside of the Ctrl, the Wind reverses the order. I've seen both before, but I never can get used to Fn on the outside. The Acer keyboard also has the arrow keys set down a little from the main keyboard, with, most critically, the pageup/home, pagedown/end keys just above. This is the way my X31 was configured, additionally with the two keys mapped to page back/forward for web browsing. It is an excellent layout choice. It also puts the right shift key in proper place directly under the enter key.
If you are going to have a wide bezel, as both netbooks choose to do, black is more attractive. Come to think of it the Wind would be nice in black. Overall the Wind is nicely designed, better looking than either the Eee 1000 or the Lenovo S10, the two 10" competitors right now.
I know its a small difference is size, but the Acer has the "awww cute" factor going for it that the Wind somehow doesn't.