I'm leaning towards the game controller boards myself. Been reading a bit and I'm seeing various issues popping up here and there with the keyboard encoders. Aside from the always mentioned 'ghosting'. I'll go for 4 of the single gamepad encoders I reckon. I've seen the 2 gamepad in one encoder boards as well but they setup P1 and P2 joysticks as being one 1 gamepad, P1 being left stick and P2 being right stick. I don't like this idea it seems like that's just asking for trouble with compatibility in certain games. Like some games may not want to let you setup P1 and P2 controls on the one gamepad. Also I'm thinking 4 single gamepad boards may be easier to troubleshoot if any buttons or anything stuff up seeing as P1,P2,P3,P4 are all isolated from each other.
The one player boards do 12 buttons and a joystick each. I'm gonna use 8 buttons each for P1 and P2 leaving 4 each free for insert coin/select and start buttons or whatever.
P3 and P4 will only be 4 buttons and 2 for coin/start leaving 6 buttons free on each if you wanna setup some dedicated frontend navigation buttons.