The ultimarc ipac I/o and the howler are both great all-in-one controller boards. I believe the differences between the two are that the ultimarc unit comes with a harness and the howler comes with screw down terminals. The ultimarc unit does not have built in resistors where as the howler does. The howler also includes an accelerometer. Otherwise, I think their feature set is the same.
I do have a like new howler I will sell. I bought a neo geo and will not continue my mame project. I updated the firm ware and otherwise unused it. Shoot me a pm if you are interested.