A splash top is like a rain jacket. It will have velcro closures on the wrists, neck and a synch at the bottom. If you swim or even get a moderate splash, it will not keep you dry at all. Think of this as a rafting only garment.
A semi-dry is built like a drytop. It's more form fitting, generally designed for kayaking, will have neoprene-like gaskets that aren't too tight on the neck and arms, and an inner and outer tunnel system for your skirt. It's generally considered to be more comfortable than a drytop, but it will let water in, though not all in a rush.
A drytop has fully sealing latex gaskets. Tighter, yes, but it really should do the job of keeping you dry unless your out for a longer swim.
Personally, I recommend the drytop if you're going to have just one option in your bag. The water in Colorado is just generally cold and if the weather turns on you, you want to be in the best shape possible. They really aren't that uncomfortable and provide a much wider useful range for kayakers.