I'll admit to having never even SEEN 1st, 2nd, or 3rd edition...sorry (which is weird, considering how large and obscure my collection can be, at times). I got involved with 4th ed only.
Let me state I love 4th ed, still...to this day. Period.
To me, that was Gamma World, as that was my only experience with it. I never bought 5th ed (Yay for me!), and have not picked up GWd20.
But I will. I liked what I saw, and I like the look of the line up of supplements.
See, my name is Shane Murphy,...and yes, I am still addicted to Fallout.
4th ed doesn't fill that need to WANT to play an RPG based on Fallout. To me, GWd20 reeks of Fallout influence. Thus,
IT MUST BE MINE!!
So, I will say I am a fan of 4th ed and GWd20 for two different reasons.
But, I think that's what makes re-imagining games fun, you can enjoy each one for something different, and game to your heart's content.
(PS I own Omega World, but have yet to really read it, sorry

)