Tybee Island is a nice place to visit, but I wouldn't like to comment on whether it is worth staying there. To be honest, in our brief visit of about half a day we didn't see a major amount of things that would keep you occupied. Of course, if it's summer you are going, then it might be a worthwhile beach destination but I would say Hilton Head would be a far better choice. The beach at Tybee is nice enough, but Hilton Head wins hands down. It all depends on what you want to do. If you are looking for a relaxing end to your holiday after the exertions in Vegas and Savannah, it might just suit you. If you are looking to be kept occupied 24/7 (you know what I mean) I would either plump for more time in Savannah or go to Hilton Head.
With regards to Savannah, Midtown isn't necessarily a bad option especially if you find a hotel on Abercorn. With a car it's a doddle journey into the Historic District, without a car, it's a simple bus ride (Abercorn is a major road through Midtown into Downtown Savannah). If you can find a deal in the Historic District I would always say go with that, but I can sense you are having the same concerns as us with regards to cost (and we got $2 to £1 last year also). I would try
http://www.Hotwire.com and utilise tthe expert advice on
http://www.betterbidding.com and see what comes up on your dates. There are no guarantees with what property you will get but you may well find a bargain. I know the Westin comes up a lot on Hotwire, and that is situated across the river from the Historic District and you can get a water taxi to Savannah which is quite cool and I think free if you are staying there.
Unfortunately a lot of the historic houses that offer rooms are very expensive. I know there is a Days Inn in the Historic District which is nothing like any Days Inn I have ever seen before which might be worth a pop, but prices were high for that even in March. The Hilton Desoto was a nice property also, but again it's all about price.
I would have no hesitation in staying in Midtown again and that Hilton Garden Inn is a lovely hotel. Just be aware that you will have to pay to park in the Historic District and this can mount up. You can get free parking at some of the tourist info centers but they fill up very quickly.
Hope this helps. Fire away with anything else and I will do my best.