I initially mounted my Zumo using the clutch reservoir mount. I've since moved it to a steering head mount. So, I've had it in both places.
My thoughts;
On the reservoir mount, it was up nice and high putting it nicely in either your peripheral vision or you could focus on it and use your peripheral vision to keep an eye on the road. It was easy to reach up there as well. I have an Audiovox cruse contol on my bike, though, and the control pad is right up there, so having the GPS mounted up there made it a bit difficult to get to the crusie control, uh, controls. And when the missus rides with me, she can see the GPS from the pillion position.
For the steering head mount, it's nice and low - out of the way of things. For me, it's more difficult to operate down there 'cause I've got a tank bag that'll easily accomodate a midget or two, so reaching over the tank bag is a PITA, and I really have to take my eyes off the road to do so. Compared to the clutch reservoir mount, the cruise control is easier to operate, but the missus can't see the GPS when it's on the steering head mount.
All in all, I like the steering head mount because it's nice and low and outta the way of things, but it's more difficult to operate. I like the reservoir mount because it puts the GPS up closer to my line of sight, it's easier to operate, but it gets in the way of the cruise control, uh, controls.
Hope this helps...