We were considering Tybee for a family vacation in the future. Glad we checked it out first! Our 24 and 29 year olds and their boyfriend/husbands would have gone stir crazy. It's very undeveloped. There are a few very small (and a little shabby ) beach bars, a couple fast food restaurants and one very nice ocean front restaurant in the "just ok" hotel we stayed in. It's the kind of place you go to do nothing! For night life you be best served to drive into Savannah.
We weren't impressed enough with Hilton Head to ever go back there either. We had the same feeling that we've heard from several others - it's too resticted. You can't just wonder where you want because it's broken up into "plantations" and if you're not staying on that particular one, you can't just drive through or explore.
My advice would be to consider staying in Savannah ( for the nightlife) and making Tybee a side trip to spend some time on the beach.