I posted the below response on the Hyatt section, but thought it might be of use as a general resource.
Here's how I see the branded hotels in Charleston:

Hyatt - The only choices downtown are the Hyatt Place and Hyatt House. They share the same building and amenities. I typically chose the HH studio and usually get upgraded to a one bed room. Good breakfast. Local news recently had a story about the properties being sold. The HP side is going to be rebranded within Hyatt to a "more upscale soft-branded lifestyle experience with the addition of a restaurant, a bar and remodeled rooms". Not sure what that means. The HH will not change. Be aware that upper King St becomes an entertainment zone in the evening (Charleston College is nearby). No real noise problems in the hotel. There have been problems on the street late in the evening. As a result, upper King is traffic restricted on Thursday to Saturday nights with a very strong police presence. Not anything to really worry about, but when the bars/clubs close, things can get kinda weird.

Marriott - The downtown full service properties are the Grand Bohemian and the Renaissance, both of which are well located. Have not stayed at the GB, but the public areas are nice. I have stayed at the Renaissance in the past, but not for several years. Stays were always good, but don't expect any upgrades. The Charleston Marriott is west of downtown on the Ashley River. It is not really walkable to downtown, but the hotel has a free shuttle to downtown area. This hotel has a very large rooftop bar that I enjoy. Views of the river and usually pretty quiet. The Hotel Bella Grace (Residence Inn) and the Courtyard Historic District are well located and supposed to be a bit more luxe than typical for their brands.

Hilton - The main choices are the Mills House, Wentworth House and the Embassy Suites. The Wentworth House is a SLH. My only stay there was as a Globalist on a free night. Not much of an upgrade. Very nice property in a residential neighborhood (about a 10 minute walk to King St) Free wine and hors d'oeuvres in the evening. They have an excellent restaurant. I don't know much about the Embassy Suites, but it very well located being next to Jackson Square. I had avoided the Mills House in the past because I thought the listed room sizes were a bit small. Stayed there about 18 months ago and was presently surprised with rooms. The hotel has a very nice bar area with an adjacent large courtyard.

There are two large independents that are quite nice ($$$$), Charleston Place and The Bennett (nice champagne bar). We are getting a Four Seasons over the next year or so.

One note about visiting Charleston in July and August. You need to be prepared for the weather. For those months, we have two types of weather: hot and d*mn hot. Most of the time it is d*mn hot.