I'll be honest with you, when I bought XI back in the day on my PS2 I played the hell out of that game for 5 years straight with a Playstation controller. So from coming from someone who loved the PS dearly, trust me when I say go for the Xbox 360 controller. The official Microsoft Xbox Controller is a little pricy (upper $40s when I got mine awhile back from Best Buy) but it is definately worth it.

For XI for PC and XIV I had used a Logitech Dual Shock PS-style controller for a while before switching to the 360 controller. And while I swear by Logitech the PS controller design is just not as solid as the 360's. Functionality wise the 360 controller's Analog sticks and Trigger buttons have a faster response time than the PS model(s?). Also one huge flaw I had with the PS controllers is that moving the analog sticks results in either standing still or running (on the actual PS2 version slightly moving the stick = walk). I fought with the PS contoller for several weeks trying to get that "walk" but I could never get it to work. The 360 contoller plugged straight in and everything worked perfectly (even my trivial desire for walking :P)

Also, its funky at first if you've never used a 360 controller, but I found it to be more comfortable than all kinds of PS models I used over the last seven or eight years.

One word of caution though: If you still play XI and use a controller for that game you will need a program to emulate Ctrl and Alt for the triggers because XI does not recognize them as actual buttons and it will not work properly. If you don't play XI don't worry about it, it works flawlessly with XIV.