Depends upon how the site runs as well. If there are gaps between games, respawns as well then you can run with mids and have time to relaod. If not then hicaps may be for you. However this is a personal choice. Most hicaps will shot about 50 bb's between winds, most mid caps will shoot 100 before needing changing, so potentially you can actually fire more bb's in a fight from a midcap at one player before pausing.
You may like to provide supporting fire to others, then hicaps are good, you may like to sneak, then mid/low are good.
Fire discipline has nothing to do with what magazine type you are running, it has to do with the player.
I would recommend you get 2 hicaps to start with. Its a low price point and will allow one to be used and another to be spare and filled. When you get a respawn/pause in game you can refill both from a spare bottle/bag in you pocket or pouches.
I tend to play with 4-6 hicaps. Don't always use them all, sometimes I do if I feel gung ho and in the middle of a large fight. I also carry about 2000 extra in a pouch to refill when I can. This also allows me to pass a mag to a hire gun, or give bb's out to those that have run out mid game (I mean 500bb's is what a £1). Also stops me going back to the safe zone to get more. Some days I will use 3000bb's, some times I will use 300, depends upon what type of day I have.
As to rattling, yes it does and does bother me sometimes, if I can hear it over me puffing like a train and tripping over logs! However it is surprising how quiet they are when you move slowly.