What an interesting thread !!!!
Perhaps I can add a few things not previously stated (unless I missed them) which may be of interest.
I evaluated a range of models and came up with the 276C, but that's because it suited
MY purposes.
Firstly, the SP26xx series have a touch screen, the 276C does not.
Personally, I prefer operating the buttons than trying to touch the screen in the right place, because I can perform a number of operations by feel rather than taking my eyes off the road all the time. But note that most of my riding is on country roads rather than city streets.
Secondly, the 276C has an internal battery, the 2610 does not.
I like to take the GPSR off the bike at coffee / lunch stops and plot my next moves or generally just review track logs etc.
Thirdly, the track log on the 2610 is just too small for my type of riding - recording at best resolution, the track log (I believe) is only good for 150 - 250 kms, while that on the 276C is around 2000 kms.
Next, the 2610 records coordinates in Lat/Long only, it doesn't understand UTM. This is probably of no interest if your only use is navigating around town, but it's more than possible that you may actually get interested in using the GPS for more than you originally intended.
My own review found the 2610 to have less capability than the 276C, but there are probably many more Quest and 2610 than 276C users, demonstrating that they suit a lot of people.
A while back I wrote a GPS Buying Guide, see it at:
http://www.bluerim.com.au/Ulysses Review.htm
It is neither exhaustive nor definitive, but it may at least allow you to ask the right questions, and be able to evaluate the quite useful information that other posters have provided.
It may also assist in sorting the wheat from the chaff.
Have fun, but really, you cannot determine which is best without understanding your own requirements, then matching the capabilities of the various GPSR's to your needs.