What's the main difference between getting the rackmount vs. the bodypack receiver?

The bodypack can be mounted on the camera. The 100 series is not a diversity system (heck, if the mic system is not, does it really matter much if the receiver is?).

Still having a better set of antennas has got to make a really good system only better. So if portability is not an issue, you might want to try a rack mount.

The wall-powered receiver is diversity. The Body Pack receiver is not a diversity receiver. THis is according to the Evolution series 100 manual.

The transmitter has nothing to do with whether the system is diversity or not.

Basically, a Diversity system is just a dual receiver setup with an auctioneering selector between them. The best signal gets put through to the output. The 'diversityness' of the system is based on having two separate radio receivers with antennas that are physically separated by some amount.

Sennheiser even allows one to extend the antennas to remote locations with extension cables and different antennae.

Thanks Mike and Nathan that helps.