I just purchased a Garmin Street Pilot III, and find that in bright sun light the screen washes out badly. Even when I shade it, the details of the screen are hard to see. I have not tried it on my R1100GS. I can still exchange it for a Garmin 2610. I have been told that the 2610 has much better visibility, but that it might be difficult to use the touch screen when wearing motorcycle gloves.
On the plus side for the SPIII: has battery capability, buttons can be operated when wearing gloves; on the downside, the screen appears that it will be difficult to read in bright sun. For the 2610, on the plus side, the screen is (I am told) easier to view. On the down side, it costs more, does not have battery capability, and may be difficult to use when wearing gloves.
I would like to hear from folks who use either of these GPS devices. What advice do you have for me? Thanks for your responses.