The simplest answer is, "It depends".
It depends on how much you want to spend. I went complelety overboard and got the s720i. The main reason I got it was the ability to download my workouts to the computer and graph them there. The software that comes with it isn't the greatest, but it does let you see time in various user defined zones in a variety of formats, from percentages to actual times. It also tracks this info daily, weekly, and I believe monthly, if thats you kind of thing. I believe that model supports 5 user defined zones, but since you have the data on the computer, you could get really creative if you wanted to.
I'll admit I haven't played with every feature of the thing, but if there is a specific question you have, I'll try and answer it for you via actual experience or the user manual.
For what its worth, I'm glad I bought it.