Getrag > ZF
To add a little to what to jess is saying the ZF also has what could be described as design shortcomings in a couple areas. The ZF's develop shift problems after time that are related to the catch pins. They actually released a Technical Service Bulletin
SIB 23 03 02 covers the shift pin replacement.
Some of the ZF's also experience input shaft bearing failure too. Friends of mine that have e36 M3's with the ZF ZF S5-31 with more than 100K on them have had both of these issues.
But, if you are into reving the engine to redline and dumping the clutch and slamming the gears between shifts you will break either of these transmissions. If that is your thing I recomend a TH400
. These transmissions are just not designed for that kind of driving, but they can handle spirited driving all day long and they are lightweight. I have had a Getrag in my car, I beat on it a little here and there, but I don't slap it around and it has held up really good. It has a ton of miles on it, and it still shifts precisely and makes almost no noise when it is engaged in neutral. I serviced it at 90K miles and used
Redline D4 ATF. That made a noticeable improvement in shifting and it has held up great. I don't think I would never go to a ZF even with an S50/S52 swap. I have been working convincing my buddy that has a '98 M3 who has gone through 2 ZF's to switch to a Getrag, but he won't he is too much of a purist...