I keep bugs and do email/support via FogBugz, but when it gets down to crunch time I always end up back in Pivotal Tracker for my weekly to do. I work alone.

FogBugz email integration is top notch, but the time tracking and staying on target for a release is definitely tuned for a larger team. In "list of cases" view there isn't an immediate visual cue to what you can get done in time for a release. Going into reports you get nice graphs with confidence curves for the finish date of a release. Entirely overkill for a small team; possibly a life saver for a larger one. A side note, I was able to show a non technical person how to use FogBugz to work with email with surprisingly good results. The person found it a very natural transition from plain email and appreciated the case tracking capabilities of FogBugz.

Pivotal feels much more natural to me and always helps me stay on track. I can immediately see what I can get done this week and what I will have to push back or revise. I also find its 3 point system for estimating remarkably accurate. The tool has actually driven me to get things done as well, which is more than I can ask of any tracking system.

So,

FogBugz: a formal bug tracking system with excellent email integration and fine grained estimation and tracking.

Pivotal Tracker: a "to do" tracking system, with coarse estimation and tracking that works better for fast iterations and small teams.