I'm planning on doing some extended touring for the first time (I've only ever been to New York, Philadelphia and one brief stop in Miami) but I'm a little unsure of where I want to go. Would appreciate getting perspectives form both clients and escorts. What are some of your favorite cities? Why? What's the best time to go there?

In addition to any suggestions you get here (on general), followup with the local boards to find out if anything is happening that could affect the success of a tour. Large cities have convention and tourist websites that list things happening weeks and months in advance; that can affect the availability and price of hotel rooms. Sports is another drain on discretionary or lack of discretion-ary spending.  If the city team made the playoffs, that might affect business, not just from those few with playoff tickets, but those hanging with friends at bars or at home to watch TV. Less time left for other kinds of fun.
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When some major big whig visits a city, it can bring traffic to a halt as they shut down streets. A major construction or repair project can screw up traffic for weeks at a time. Check with the local boards about things like that, too. And, of course, there are travel and weather advisories, especially in the hurricane zones right now.  
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Yes, you'll get lots of good city suggestions and requests, but follow it up with some extra homework.

where you can do the best business, or which cities are the most fun to go to?

Possibly go to every major city.

At least that I have visited so far. There are a lot more on my list.  

Love these two cities !

I always do great in the only cities I have toured in... Nashville, Chicago, Cleveland. I frequent these cities as well, in fact I am doing my three weeks in Chicago right now.