I'd have to go with Lost Maples. It seems that Hill Country is more suited to horseback riding (I was dodging them all weekend) than hikers. The backcountry campsites were good in both, althought I do like the Wilderness camp area in Hill Country best 'cause you can have a fire.
As far as scenery is concerned, it's practically a tie between the two. Both offer some really neat - and different - scenery and they are the only two parks between Austin and San Antonio that I would consider revisting. The edge would go to Lost Maples during the Fall.
Water access? Hands down Lost Maples. There were times in Lost Maples that I could have easily thought that I was in the northeastern US with all the water that was around. The pools in Hill Country, while looking fine, were questionable to me with so many horses visiting them.
Bottom line, I would give a slight edge to Lost Maples. Overall I'd say it's probably the best place in central TX for an overnighter.