Well the stick needs to have a sustained write capability. Maybe check for sticks that can do the windows vista / 7 application speedup (Windows Ready Boost). That should be able to sustain high and consisten read/write speeds.
I can't suggest to use a SSD for timeshift. It just has too many writing operations, and a SSD is more the "please just read from me" type of disk. In a time of about 8 months my SSD (SuperTalent GX2 64GB) died, just because it exceeded the maximum write operations. This was in a single station setup, with multiple clients this would have happened much faster. There may be better SSDs out, but they cant make it for years.