The Acer S3 has a 1366x768 display and weighs 3 lbs. The Surface Pro has 1920x1280 and weighs less than 2 lbs. Also, the S3 is being criticised for not having a particularly good screen, whereas the Surface RT has been widely lauded for having an excellent display, so the Surface Pro is bound to be very good as well.
Those are pretty significant differences, and for some people can justify spending the extra money. I know for myself I'd refuse to buy a tablet with 1366x768 resolution -- that's less vertical resolution than I had on my Windows 95 desktop in 1996 (!!) ..... and let's not forget that the iPad offers 2048x1536 resolution.