Having flown with Garmin, Rockwell Collins, and UNS on a variety of Citations and Mustangs, I prefer the Garmin.
Collins is a true "airline" system, orientated to purely IFR flying, with multiple redundancy of position information, calculated from all possible navaids.
Garmin only uses GPS for enroute navigation, but this is good enough these days. Where it is excels is in mixed IFR/VFR flying. I find it a big aid to overall situational awareness compared to the Collins.