"Touring" covers so many things it's hard to pick one best.
Something like the Garmen 800 or Edge might be better since it shows you a map. The 500/510 may be able to follow a pre-defined route, but if you get off track, I don't know of a cycling GPS that'll reliably get you back. You can get some flaky routes if you let it re-calculate for you. You looking at a map can usually do better.
For a multi-day trip with camping, the etrex series might be better because you can buy AA batteries to keep it going. If you're doing motels or a B&B, the Edges can be recharged overnight.
My electronics strategy is to stay at least one version behind state of the art. From what I've read, the Garmin 1000 is exemplifying why -- let the early adopters muck with it for another year or two, and they'll get most of the bugs worked out.