I'd love to hear some admins of private trackers comment on this. I've been told by a couple major ones that they only keep 'essential' logs, everythign else is 'wiped'. With that said, they also value storing IPs so they can block unruly users. I'm not sure how much I trust these admins.
I think private trackers are safer more because you are not involved in the massive torrent scene when it comes to music movies games etc. When you torrent from the pirate bay, you are by definition leaking your IP address in the most public way possible. MPAA et. al. track those major torrents.