Looking at the configuration you are going to do (without seeing a picture of the layout you proposing), you might think about a future upgrade to a three player cab.
I'm not sure what you meant by (2) 4-way fire buttons (shared w/player 1); but, if you are talking about firebutton JOYSTICKS, you may want to have them separate later, to allow more players--or to support multi-player, multi-stick games like SmashTV.
If you are talking about just 4 fire buttons (ala Vanguard), you could swap out the 4-way only stick for a T-stik later on, and support a third player using those buttons.
Anyway, if this is a possible option down the road, I would definitely go with the I-pac.
As Tiger-Heli mentioned, they can be daisy-chained to provide support for two more players.
I'm not sure if you can run a KeyWiz and an I-pac together.
My thought is that inputs from one will disable the other temporarily, which would make the whole system unworkable.