Unlike JeffB, I am a much more casual Runequest fan; I've played 1/2 and 3, some Heroquest (of whatever it's called), and the Mongoose Glorantha game. I've browsed the RuneQuest: Roleplaying in Glorantha book, so take my opinion with that in mind.
I'd play a Glorantha game in 1/2, a generic Iron Age game with 3rd as I don't think it's as suitable for actually playing in Glorantha, Mongoose is serviceable, I would never play the new one because it's mechanically so hard to understand I sort of gave up. I always thought the magic system for RQ was best in 1/2 and went sort of downhill from there.