@gooch, true that!
In Texas, a ride that distance in hilly terrain between May and October (temps range from 90’s to 110 and even beyond) can easily dehydrate a person to the point of incapacitation. We’ve seen people die on the trail here.
It also depends on your level of fitness and experience. If you’re Lance Armstrong then it wouldn’t be an issue at all… but then anyone with anything near that level of experience wouldn’t hafta ask. ;-)
That said, get the cheapest one you can find. If you are in a really hot area or planning extended distance, consider upgrading to the cheapest 100oz. size.