Haven't owned either, but this is what I've gathered from here, the net, and gun rags:
Weight: G3 runs at 4.5 Kgs. The FAL is trickier because there are so many different variants, but
http://world.guns.ru claims 4.45 kgs.
Reliability: Both are supposedly very, very reliable- IE, drag through the dirt, AK reliable.
Maintenence: Both can field strip into major components easily
Cost: A real HK-91 will be +$2000. Clones run from $400 to $900. A DSARMS standard FAL (built from an original parts kit) will be about $900
Accuracy: Battle-rifle accuracy. 3-4 MOA or better.
Aftermarket goodies: HK-91 has tons of scope mounts, but so does the FAL. You can get the FAL with rails and such.
From discussions I've read here, the FAL is generally deemed to be more ergonomic. A better safety location, ambidextrious (sp?) magazine release and bolt release. The G-3 doesn't have a last shot bolt hold open, and when you recock it you have to overcome the roller-locking mechanism and the spring. Also, the charging handle is way out front. Run a search for tons of discussions on this.