No car rental counters at the Bradley Airport. All operators are off site and use shuttles. The rental locations are only a few minutes away. You can stay at the Sheraton then take a shuttle the next morning to pick up the car. As you mention, you could also stay at the HIX and pick up the car from Dollar the next morning as it is right next door. There's also a Candlewood Suites that is walking distance from the Enterprise location as well.