Both, Sukhothai Historical Park and Ayuttaya city are UNESCO world heritage sites, featuring grand ancient ruins. Ayutthaya‘s location (info on getting there here) makes it a convenient day trip if you’re in Bangkok. The ruins feel more varied in the way of chedis, wats and palaces, with a few grand buddhas sculptures to make you gawk. If you’re a fan of Buddha pageantry, with the exception of the wat (photo below), there’s less of them in Ayutthaya. But Ayutthaya is also a city and its sights are spread out, so it’s ideal to hire a taxi to get around to all the different sites, since it’s also hot.
Cons: Ayutthaya is a bit more of an arid area and the “hot” part, I didn’t enjoy. Also, having to drive around to get between sites wasn’t as fun. You either need to know exactly which temples you want to hit or trust the driver to take you to the best ones. Each wat has different admission fees, so your visit could add up. It’s easier if you can go with friends to split taxi costs and it will take an entire day.