I would take a private tour with air con car, guide and driver. Ayutthaya is baking hot and its spread over a large area. You'll really appreicate that air con as it heats up. It can be 40+ by mid day. Also, if you're in a car with your own guide and driver you can linger more over bits you like and cut short bits you're not so keen on. Its a hassle free way to see Ayutthaya.
We went with a company called SalaThai travel (we googled them). we've been with them on a number of tours in and around bangkok. They have a brilliant website and what you see is what you get. They have about four different Ayuttahya tours.We plumped for the full day summer palace and ruins tour, which was excellent but exhausting. With hinsight though If I'd known how hot it was I would have gone for the half day ruins tour followed by a cruise down river back to Bangkok.
It cost us (2 years ago) 3600 bhat for 2 of us and that included the car, driver, and tour guide, all petrol, entrance fees and bottle water and cold towels and lunch. We found it to be a lot cheaper than other companies and
its one price whereas many private guides quote a price and everything else (transport, petrol site fees etc) is all on top. We're also quite fussy and have been to Bangkok 7 times now so we like to tailor make our own tours and Sala Thai are very amenable and will add or take off bits from the standard tour at no extra cost. The guides have always been very knowledgeable and speak good English.