It has been a few years, but I took the tour from the Shangri-La hotel. I liked it especially since there were only 15 people on the tour in a big bus. Easy to spread out or move if the Sun is too intense on one side. The main reason to visit Ayutthaya are the temple ruins although you can ride an elephant for extra baht. I really enjoyed the cruise back to Bangkok and the buffet lunch on the boat was good.
(I saw the cheaper tour groups crammed into buses and was glad I avoided that fate.)

If you are tired of temple ruins then give it a miss. Plenty of non-temple activities to do in Bangkok.