I have used both the u360 wiring harness which works great. I have also used an Ipac. The Ipac has keyboard functions and firmware that you can custom program to suit your needs, including a shift key that doubles your input power. The u360 wiring harness inputs to your system computer as joystick type input. It isn't a big deal to program within mame to desired key input, but with the ipac this is done by default. This makes the Ipac a little nicer IMO. However, it is a pretty minor inconvenience to save a few bucks.
Neither really works *better* than the other, as they both perform excellent and are reliable.