A GPS is a stress reliever. It doesn't take the place of a map, instead, it compliments the map. You use the map to determine where you want to go, and you use the GPS to tell you where you are on your progress towards the destination.
I have used a variety of them - StreetPilot III, GPS 276 and 296, SP 2650 - they all work well. If you are planning a round the world trip, and don't plan to carry a computer, then get one of the SP 26xx series GPS units. They have a much bigger storage capacity for maps than the SP III or the GPS 276. This will allow you to load up all your maps before you leave, then you won't have to fool around with that issue when you are on the road.
Functionally, the SP 26xx series and the GPS 276 are quite similar. There are some differences, but for your level of experience, they are not significant. The SP 2610 is the 'non BMW branded' equivilent of the BMW Navigator II.
Keep in mind that the cost of the maps can be quite significant for a RTW. The GPS unit will come with maps for one continent (e.g. Europe), you then have to pay to buy maps for other continents (e.g. North America, Middle East, South Africa, Asia, Antipodes). For motorcycle purposes, the "City Select" maps are just as good as the City Navigator ones - exactly the same visual content - and quite a bit less expensive.