How big is the difference in gameplay between 2.51 and 2.74?
The difference is on single high health elites. Usually you can keep them where they belong, under your twisters procing instead of running and thus you dont have to start wind-up all over. 3 is even better, many times they can't even even spit out nasty affix's like sentry or desecration - but most of us choose not to care about that and weather their affix cos we go tank which is easier at 2.73 than 3.
IMO 2.5 is a non starter. Find something else cos chasing things all over the map gets old quick not to mention when you're not procing you're dead in the water. Shell's not reseting killing suvivalibity..