i have experience with the g2 ew100 and the previous generation ew-500 and the construction on the g1 was a bit better/heavier...the cases of the transmitter/receiver were metal vs. plastic for the g2, although the case on the g2 is a little bit smaller/shorter.
but in use there wasn't much of a difference except the g2 added the pilot tone.
Matthew, there are two significant differences in addition to the pilot tone - one good thing on each. On the old "g1" it had removable antennae, and on the G2, it runs on AA batteries rather than 9V.
Difference in 100, 300, 500 is minimal for a normal shooter. Obvious differences are things like charging contacts on the outside of the body packs... so that you can drop them into a Senn charger. Personally I think these are good enough to have a bunch of them, but until you do there's not much extra value in the higher models.
The 500 also has things like built-in EQ and stuff like that... I've got a 500 bodypack that I use interchangeably with the 100 and I don't really notice any performance differences.
It always amazed me that Sennheiser failed to put any defined performance or usage differation in the higher models - I would have thought things like detachable antennae or metal antennae would have given the person paying the money more value for it.
The only notable difference is with the plug on Tx, where only the 500 version offers phantom powering.