Partly to achieve the number of stays required to requalify for Starwood Platinum and Hilton Diamond, we stayed one night each at the Sheraton Suites Key West and the Doubletree by Hilton Grand Key Resort.
Both are fine. If you want to stay at one of the "name" chains elsewhere, if you have to ask the price, you can't afford it.
The Sheraton checked us in promptly, upgraded us to a "pool view" suite, and allowed us to check out as late as we wanted to. The hotel is immediately across the street from the beach.
We checked in early at the Doubletree and our room happened to be ready. We were given a Doubletree cookie and two bottles of water, along with a voucher for a full breakfast buffet in the restaurant. We were also "upgraded" to a pool-view room.
The Sheraton suite was more spacious; the Hilton room was somewhat more updated.
Parking fees were waived for us in both properties. Both have a free downtown shuttle but there wasn't room for us on the Sheraton shuttle when we would have gone. The Doubletree shuttle, on the other hand, guarantees a space.
Either is fine if you're trying to qualify. All other things being equal, we'd give a slight edge to the Doubletree, but would willingly stay at either property, depended on on price.