To be fair, I've done both. By now I can roughly estimate that a task will take me one hour. If I dedicate myself to it right now, it will actually take me 2-3 hours. I can also sort of carry it around for a couple of days, and then implement it in one hour.

This says two things about me: I usually underestimate the design details when estimating time, and the design takes me roughly the same time anyway.