This conversation has been beaten to death. Fixed roof vs. removable roof. Due to the change in RT/10 configurations, there's actually a new 3rd choice. I've had both the "targa" version (Gen 2) and the convertible version (Gen 3/4 SRT).
The decision is actually far simpler between the first 2 choices. If you plan to track your car much ("much" being the operative and decisive word), the you should own a GTS (safety, stiffness, track specs, etc.).
If you're not tracking your car, then I can't understand why you would want a fixed roof (unless your toupee glue is is beyond it's expiration date) ... which brings me to the choice of removable roofs.
The Gen 2 targa (hard top) roof (sorry, but the OE soft top is hideous looking and should be used only in the case of an "emergency" hurricane ... it looks so inappropriate that I'd hesitate to use it even in a rainstorm preferring to get wet than be seen with it in place) effectively makes the Gen 2 RT/10 into a coupe. It's even accepted by most tracks for road racing. I actually miss this option now that my roof is attached. The trade off is the exactly that, having it attached. With the Gen 3/4, you get to make the open/closed roof decision while on your cruise, rather than in your garage before you leave. Best of all worlds would be a OE hardtop that can be stowed in the car. There are some aftermarket stowable tops, but that's not what this thread is about.
There is simply nother better than having the ability to cruise ******* in the AM on the first half of a long drive and then put up the top for an A/C cooled drive home when the temp becomes unbearable in the afternoon. When the day (or often even better, the evening) is gorgeous, I'd kick myself silly having to sit inside a GTS. It's just a completely different driving experience. Top off/down and you're SO much a part of the experience, even a relatively low speeds. I have absolutely no actual info, but I choose to believe that many fixed roof owners (that don't track) make that choice having never experienced the road in an open car. Could very well be wrong, but it makes no sense to me to have a fixed roof car for street use (i.e., no tracking or street racing, etc.).
The only other factor is the "look" of the GTS rear end. I believe with the Gen 2's, the GTS rear is much better looking, while in the the Gen 3/4's, the difference is less dramatic. Nonetheless, when I consider the choice of the "rear end look" vs. the ability to remove/drop the top, the choice is obvious.
But of course, that's just MO.