Our first trip down (2004) we brought our seats (infant carrier, which she used on the plane and a regular car seat for older dd). It was a pain to carry, but was worth it as when looking at the seat they didn't look great and not a great selection. This was through Alamo.
On our next trip (2005) we used a regular carseat on the plane for youngest, which was a good thing because she would nap in her carseat. So for her it was just like being in the car (she was 17 months old at the time). We rented a booster for oldest dd who was then 4 years old. This was National and they had a great selection of seats to choose from.
Our last trip (2006) we rented a carseat for my youngest as sitting her in it on the plane would have meant kicking of feet right against the seat in front of her (she was 2.5yrs at that time). I actually found the seats last year to have been better than any of the other years. They were wrapped in plastic, unless someone took it off to check the seat and then they left it laying there. You could smell the cleaner on the seats, so obviously someone is cleaning them. We actually had a really nice seat we chose for dd which was a nicer carseat than we had at home (can't get this kind here). For older dd I carried the backless booster in a duffel bag and we used that.
I guess it all depends on the rental company and your preferences. Lugging two seats with twins on your own, wouldn't be "fun", but would save you having to rent them. Through National it costs $9/ day, but maxes out at a cost of $49 ... so if I decide this trip to rent for youngest it will cost me $49 for 10 days. I don't know how other companies calculate their rentals.
Good luck with your decision. If the twins are young and would sit better in them on the plane, I would take them. But if you don't have seats purchased for them, it depends on your preference level.
Kerri