Never had 2836, but from my memory:
2836: gmt hand sweeps and sweep set, some people had issue where the gmt hand would occasionally slip, so if they kept it on a winder after a few days the gmt hand might be a 1/4 hour off. Not a huge deal, some people have said the extra gmt mod makes a the 2836 a little unreliable in the long run but I haven't heard of that many people with problems. This is readily available and at a decent price.
289X: Same base as gen, jump hour... But quite a few were set with the gmt hand a little off the hour mark (so at 12:00 your gmt hand would be pointing at 12 1/8. Because of the jump hour set you couldn't fix this without having the hands removed and reset correctly. This model with correct hand set (the reps hands are too long), a new datewheel (there was a project over at rwg dont know what happened), a new date mag, and a better crown, would be perfect. This model was not available from dealers 6 months ago and resale was VERY high (not sure on current status)... I haven't seen one listed in a while (but I don't pay close attention as it doens't intrest me anymore).