I have an MB E350 with a hard drive and 6 CD player. Over 1,000 songs have been loaded into the hard drive.
My wife's Nissan uses the USB stick.
A lot of same songs have been loaded onto both. In my opinion, the MB hard drive gives a better sound reproduction than the USB stick. In addition, it is easier to use as it is built in, whereas the USB stick has to be inserted. You can leave it in all the time, but hen it is like a "sore thumb" sticking out. Also may get broken off. I have had them to just die and lost all the recordings.