Get a prepaid SIM card from 3 (www.drei.at), they have the widest 3G coverage. Especially if you are staying somewhere outside on of the provincial capitals I would suggest 3. Bob is the discount brand of Mobilkom Austria, they have the second best 3G coverage plus EDGE outside their 3G coverage.

tele.ring is the discount brand of T-Mobile, but the 3G network is not as good. Yesss is the discount brand of Orange, they have the smallest 3G coverage of them all.