I have been at Long Beach Harbor for 4 years now. Much of it has been redone. Piers, bulkheads, paving, launch ramps, etc. The harbormasters office is brand new. It also has a shower and bathrooms with a/c which are clean and well maintained that are available to slip renters. Each pier has a locking gate. The staff is friendly and walks each pier daily (appears morning and night) and will report anything unusual. I have taken many 2-3 night trips and never had a problem with leaving my truck there. Ice, new fuel dock and pumpout station. There is a bait shop, Steve's Marina restaurant and the Long Beach yacht club adjacent. In my experience, I can highly recommend it without reservation.
Pass Christian, as stated, is more of a working harbor. They are undergoing a major expansion that should be completed next year.
http://www.wlox.com/story/16230332/pass-christian-harbor-gets-33-million-expansion.
From link to 2010 story:
The work actually started three weeks ago. In just 16 months, a new 84,000 square foot harbor will accommodate more than 500 vessels with deeper channels, sea walls, massive parking lots, a comfort station and ice house.
Should change the face and culture significantly. Pass Christian yacht club there, and active sailing community also.
Not familiar with the Gulfport harbor, but it opened recently. Completely new and top notch from all appearances. It is pricey in comparison.
Best of luck.