Between those two, for vocal tracking, I'd go with the TLA-50. I had the RNC, but it really didn't do much for me. I liked the super smooth mode (or whatever it's called), but for the most part, it made everything I ran through it sound smaller. I got rid of it. I have and enjoy the TLA-50, and I especially like it for smoothing out and slightly fattening a vocal.
Another great vocal comp to consider is the Safe Sound P1. (The P1 is a channel strip, but you can just use it for the comp if you want.) You might be able to find a used P1 in the same price as a TLA-50.