There are lots of rumors floating around with the galaxy mega series. Sounds like the mega is a mid tier version of the note and should be available in a few months. I am thinking the note 3 will release later on in the fall.
I would get a gs4 now rather then waiting 6 months for a note although maybe the mega will be good if its priced right. again its a constant waiting game with nothing but rumors to go on right now. As far as 5.0 is concerned the gs4 will get it no need to worry about that.
For me its mainly about removable sims... if the gs4 has one I'm in otherwise I'm waiting for the next round.