I have a ASUS Transformer (TF101) and I really like it and i'm glad I bought it! The keyboard isn't as good as a convential keyboard but it is very useful for typing up notes and providing the extra battery power.
I have a iPhone iOS and a HTC Android phone, in terms of the OS, I think they both have thier plus points. I would recommend getting in front of both and seeing which one you prefere. I personally liked the Android, hence I got the Transformer. To be honest, I have used the iPAD 2 as well and I really liked it also. I'd have to get a bluetooth keyboard with it though.
I don't think you can go wrong buying either, but at the moment i'd give the Transformer the edge over the iPAD, though that's because of what I tend to use the tablet for..