Prices on Menard website:
Rapidair 3/4 line $1.50 ft
type L copper 3/4 pipe $2.51 ft.
Rapidair 3/4" 90 elbow 18.99 each
Copper 3/4" elbow .89 each
Rapidair 3/4" male adapter $15.99 ea.
copper 3/4" male adapter $1.69 ea
50 ft run 4 - elbows, 4 male adapters
Rapidair = $177.06
type L copper = $135.82
No brainer for me, type L copper. Better heat transfer and nice straight runs. Parts for future modifications are available most anywhere, no need for another special fitting.
Sweat soldering is a very simple procedure, so don't let that stop you from using type L.
DO NOT consider type M copper.
Line size is determined by air requirement at each outlet, length of run, and number of fittings. The size of the tubing for volume in the system is so minimal as to not be perceivably different. If you want additional volume find a used air compressor that doesn't run on Craigslist and use the tank and relief valve from it for more capacity.