Well, every viewer has certain features that may or may not work for you. Luckily we have choice.
Personally I use Phoenix because I am accustomed to te V1 interface and like the functionality. A few functions come to mind:
Long-distance radar
Using ":" instead of "/me " for RP'ing
Spam protection
Phantom option
Copy & paste for object coordinates (pos, size, rot)
I guess there may be a dozen more functions that I have become so used to that I would only notice if I don't have tem anymore.
Having said that, any viewer works which you get along with. Choice is the important thing!