Grandad:
There is a really remarkable difference between the SP III and the newer 2610/BMW Navigator II. The processor in the newer product is much faster - about 10 times faster - as a result, if you have a route active and decide to make a detour, the recalculation of your new route is almost instant, there is no waiting.
The screen is brighter, more durable, and there are fewer problems with reflections, because a better technology is used for anti-reflection. The screen brightness is also automatic, based on ambient light level detected by a photocell. At high magnification levels (street level detail), the newer units draw the streets with a fatter pen line (software 2.95 and higher), this makes it much easier to view the streets if you are old enough to be a Grandad. Also, the arrows used in the 'next turn' pop-up screens are fatter and provide more clear guidance.
The new products (26xx and BMW Navigator II) have much larger memory capacity, this means you can do a tour from the UK to Bratislava and back without ever needing to reload the data chip. They also have the capability to do more software tricks (e.g. avoid specific roads, avoid certain areas of town, and other tricks that are coming down the pipeline soon) than the SP III could do. Not sure if you are into this level of enthusiasm for GPSR use or not, perhaps this is not important to you.
Concerning the 26xx vs the BMW Navigator II: BMW has done a lot better job of 'adding value' to the Navigator II than they did with the original Navigator. There is an additional button pad provided- this can be quite useful on a motorcycle - they provide you with a much bigger data chip (twice the size of the 26xx data chip), this is better suited to long distance touring - and they provide a wiring harness (at extra cost) that is specific to the BMW model motorcycle you drive.
My opinion is that if you plan to do long tours - outside of the UK and deep into Europe - you should consider upgrading to the 26xx or the Navigator II. If you plan to stay mostly in the UK, and you are happy with the SP III, then save your money.
Have a look yourself at a BMW Navigator II, preferably one that is hooked up on a moto and functional, not just one sitting in a box. That way, you can make your own decision about whether you like the button pad or not. If you do plan to make long tours, then the bigger data chip that comes with the BMW unit will save you about USD 200 that you will have to spend to buy another data chip (adding up to the same size as the one that BMW ships), so deduct $200 from the price difference when making the comparison. If you decide to buy a 26xx, the Garmin motorcycle mounting kit (bracket and wire) will cost you USD $50.
BMW does provide a two year warranty, compared to Garmin's one year warranty - however, as with most consumer electronics, if they don't fail in their infancy, they usually last forever, so I don't think the longer warranty is a big issue.
I owned a SP III from Spring 2001 to Summer 2003, and switched over to the a pre-production prototype of the SP 2650 in the summer of 2003. I was very favourably impressed with the 2650 from day one - recalculation speed and screen redraw speed were the biggest benefits I noticed. But, if I had to pay the prices you poor sods in the UK have to (mostly tax, I think), I might have thought twice before upgrading, were it not for the fact that I needed the additional memory capacity (I use a 1 gig CF card) for the long distance touring that I do.
About this R1200GS motorcycle you have - is that a dual-sport (off road) motorcycle? If so, I suggest you NOT get the new 2620, which is a 2610 with a hard drive, rather than a CF memory chip in it. Long term motorcycle testing of the 2620 to determine the shock resistance of the hard drive has not been completed yet.
Hope this helps you decide.
PanEuropean
