My $0.02
I thought about both and decided on the RT/10. It leaks. I drove it once in a really heavy storm and it was miserable. Even the hardtop only helps a little. I don't care what others say, but mine isn't practical as a daily driver. The GTS is actually useful as a faily driver, but I just miss the wind in my hair. I have aftermarket windows that lock, so that helps a bit.
Ironically, the configuration that I use most is the RT/10 with the hardtop. Doesn't look as good as the GTS (which might just be a top 5 car of all time when it comes to styling), but still gets a fair amount of air when you put down the windows and open the rear window. Plus the top takes close to 45 min to change and I am _really_ lazy.
Like other say, it depends on what you want. If it's a toy car, I just love the RT and have always been partial to convertibles. If you really intend to drive it, probably the GTS, but keep in mind that those huge tires still make it a liability in the rain.