Here's a simple way to approach the decision making process:
1) Do you want the GPSR to tell you only where you are, or do you want it to also tell you where to go? By this I mean do you want to have autorouting?
1a) If you don't want autorouting, then there are many, many models to choose from, starting from about USD $80 and up. Stop reading now, and go shopping.
1b) If you do want autorouting, there are only 3 Garmin (automotive) models that provide this feature: The GPS V, the SP III, and the SP 2610 and SP 2650. (Some aviation models also support autorouting on roads, but there would be no point buying them unless your primary use would be for aircraft navigation - they are quite expensive).
2) If you do want autorouting, now the next two questions are: Do you want a large colour display or would a smaller black and white display be suitable? Do you want your unit to provide you with voice guidance for your upcoming turns?
2a) If you don't need colour and you don't need voice guidance, then the GPS V might be most appropriate - it's black and white and silent.
2b) If you want colour and/or voice, then your choice is between the SP III and the SP 2610 or SP 2650.
3) How far do you plan to go on your trips? Not the trip of a lifetime, but your average "two or three times a season" long ride?
3a) If the answer is "not farther than a day's ride", then the GPS V might be a good choice, note that it has only 21 megs of memory for storing map detail (about enough for a region containing a population of 5 to 7 million people).
3b) If the answer is "about a one week ride", then you need to get something with more memory. The SP III will hold up to 128 megs of maps, enough to cover, for example, Manchester to Geneva and back, if you are selective about what map segments you include on the chip.
3c) If the answer is "transcontinental", then you need to seriously consider getting a SP 2610. It can hold all of Europe or all of North America on one data chip (a compact flash card). You could, of course, do a transcontinental trip with a SP III, but you would need to bring a laptop PC along with you, to periodically reload the data chip with a new set of maps.
4) Do you want the latest and greatest technology? If you are a technology aficionado, then get the SP 2610. It has a lot more bells and whistles than the SP III, with more bells and whistles coming in the form of future software enhancements. If you are satisfied with something that just "gets the job done", then you can save a bit of money by getting the SP III, provided you don't need the additional touring range that having more than 128 megs of memory (the SP III's maximum) will give you.
The SP 2650 is a SP 2610 that has an acoustical gyro in it (a yaw rate sensor) and provision for speedsensor and backup light input. This provides you with precise navigational guidance even when the signal from the satellite constellation is lost, due to dense forest cover, urban buildings, or tunnels. The 2650 has not been fully proven on motorcycles yet, though - there are still a few issues to work out related to leaning into turns. But it is amazing to see it work in a car.
For what it's worth - when I go touring, I usually leave for 4 to 8 weeks at a time, and will normally go at least cross-continental, if not inter-continental. I use a SP 2650.
Hope this helps,
PanEuropean