(I hope I'm not the only PSP hacker here)
Basically, Xlink and PRO Online are both data tunelling programs that allow you to play local PSP multiplayer games online with people through wifi.
You [if you're a PSP hacker] probably already know this, but Xlink requires additional equipment but offers a HUGE compatibility list.
PRO online is a WIP (work in progress) alternative to Xlink being developed by hacker Coldbird and the PRO Team. It has a very short requirement list...
But it's still under development, and thus has a tiny game compatibility list compared to Xlink.
So, which do u prefer?