Here is a link to a comparison:
http://www.realflight.com/comparison.html

I have both FMS and G2. SInce you are looking for honing 3D skills, I think G2 definatly fits your needs more than FMS. For rainy day flying, FMS would probably do ya.

Features wise, Lite vs Full. I really like being able to customize G2 to fit as close as possible to my real plane. The customizations to do this are not there in the lite version. For example, I was able to customize the Dazzler to fly real real close to my SOmething Extra. And I was able to customize the ElectroStreak to match my KwikE. I use these customizations to practice in a crash-safe environment . To me, this is the biggest limitation of Lite vs Full.

I also use the Flight Recorder, not present in Lite. This is more a personal choice on my part since I am writing a program to analyse the recording files to use for pattern flying analysis.