We liked Foxwoods for the activities and shopping.We liked Mohegan Sun for the atmosphere and food. Both are great day trip options or if you want to spend the night. With them being so close to each other you can easily do both in one day.
Foxwoods is currently the largest casino in the US and the 3rd largest in the world. It has 4 hotels and several gaming areas. Mohegan is smaller and has 1 hotel and 1 gaming area. Foxwoods has the Grand Theater which seats 4,000.