Avoid WD my passport as I've heard many complain of the drive being slow compared to other USB3 drives.
Both Seagate Freeagent GoFlex & WD My Passport are USB3.0 compatible, compact (2.5") & powered thru USB, i.e. doesn't external power cord. I haven't used WD, but satisfied with Goflex.
Not at all. I got 100-80MB/s write speeds in my WD elements.