Can only think of 3 types of mechanical GMT/second time zone watches now. Please chime in if more options pops up in your head.
GMT function in a watch is my favourite and most useful complication.
A. GMT/UTC
24 or 12 hour
B. Worldtimer
C. Aperture GMT indicator
As I only have 2 watches in option A, I would strongly suggest to get one which have an independently detached main hour hand a'la Rolex GMTii vs a ETA 2893/7750 GMT calibre (unless your intention is only to track a second time zone in your own home country)
It all boils down to how you would use the second time indicator and your preference/your head register the time at a glance. I always prefer the main hour hands to show the local time as I travel often. If you just need to track football kick-off time/stock market opening and etc..in other countries, then a detached hour hands would not matter much.
The 2 that is currently in my collection.


Looking to add this beauty to my collection if the lust is there once I strap it on my wrist. Would try it out in Wempe the next time I am down in Europe.

Check out the cool video
http://www.hodinkee.com/blog/2011/9/6/a-week-on-the-wrist-the-nomos-zurich-weltzeit.html