If you're going on long trips and plan to have the GPS on all the time, you will want one that takes AA batteries. The Garmin GPSmap 60csx does fine, though you will also want the MicroSD card, the maps, etc., plus one pair of AA batteries per day (because most any GPS will go through batteries at that rate).

If you're going to use the GPS infrequently (turn it on, get a bearing on where you're at, and then turn it off), and you plan to have somewhat regular access to AC power, then that opens up your choices. The latest Garmin Nuvi firmware has a bicycle navigation function, and the roads are all built in (no SD card or separate maps needed). Plus the Nuvi can give you directions to amenities on route (like convenience stores, restaurants, and so on).

I've used both a Garmin GPSmap 60csx and a Garmin Nuvi 350 on tour, and they both work pretty well for my purposes.