The two are different. The Garmin is more "user friendly", but the OEM Nav has the benefit of being built in, having a nice big screen, the backup camera, the phone integration, the menus for all the vehicle options being right there. I have a Garmin too that I use in rental cars and such when traveling, I personally would never even consider buying a Lexus and sticking that ugly Garmin up on the windshield.
Very happy with the Lexus nav, have been for going on 8 years.