I don't spend extra for the retreat, so can't comment on that.
Beyond is 30 yards (90 feet) longer. It's very noticeable on the pool deck.
The for rent cabana's are facing the pool deck and not towards a walk way and the sea.
Beyond has a much, much nicer aft deck 15
The Rooftop grill on Beyond has more wind barriers, helping reduce wind while dinning.
The running track on Beyond is less intrusive to guests not using it on Beyond (despite being a longer ship the running track is smaller on Beyond, an improvement in my opinion.)
There are 2 infinity pools aft deck 15 starboard, Edge/Apex have none.
There is an additional bar (Mast Bar) on deck 16 starboard.
There appears to be an area on deck 16 starboard above the pool deck where a band can be set up (the beyond cruise I was on could have held funerals on the pool deck) so it was not used.
Le Voyage is only on Beyond (best meal I had)
World Class Bar is also only on Beyond (I had one drink there, never went back)
The Martini Bar has double the seating around the bar vs. Apex and the extra 90 feet shows up there as well.
The Club has windows on Beyond, Apex does not. Feels like a dungeon on Apex.
The seating in Fine Cut extends all the way to the edge of the ship, no so with Apex.
There is an additional bar (Oceanview Bar) by the Pizza station. Apex does not.
Beyond has a special chocolate place (not cheap) in the area leading to Eden.
There are still only two elevator banks on Beyond, just like Apex, there should be three or more evenly spaced out.
If I think of more I'll be sure to continue to add to the tread 🙂