Well, the arcade only comes with a 256mb built in memory card, theres some great classic FPS games coming to xbox live arcade soon (games like Perfect Dark N64, Serious sam HD & Doom 2: Hell on earth) and theres already great FPS games on there too like Doom and Duke Nukem 3D.
The best choice without a shadow of a doubt is the pro purely because if you intend on buying any live arcade games you will need the space in order to download these classic shooters.
On the retail front.......... Exclusive FPS games i have played on 360 and my personal score for them..........
Halo 3...... 9.5 (An absolute classic that just keeps giving and giving, unrivalled support from bungie and maps released regularly)
Prey......... 8.9 (a truly underrated game, lots of fun elements like walking up walls, cool weapons and a lengthy campaign)
Quake 4........ 8.5 (sure, theres occasional slowdown, but this game is fun fun fun, responsive controls, again cool weapons, fast slick changing between weapons and a long campaign)
Frontlines: fuel of war.......... 7.5 (not a bad game, lots of gadgets to use and control like mini helicopters and mini vehicles that explode, downside....... no one plays multiplayer anymore, kinda clunky controls)