I think personally the big download for gamma mill is worth it since it is just a compilation of other maps put together instead of downloading maybe 3 separate maps.
I do agree with you on the 999 jump since those are already inside gamma mill and so are the neji jumps.
Chocojumps is just trick jumping on maps (like oasis, fuel dump, rail gun, etc.) and as an Allie you can spawn at the very last spawn to just mess around instead and if my memory serves me right you can do some of the following things:
-Target practice with clay disks flying in the sky
-Target practice with a sniper sniping Axis at the Fuel Dump
-A timed run across a course
There are two more, though I can't remember them; if the two other maps I suggested don't get added please do add Chocojumps.