I'd buy the Nuvi for half the price. And as mentioned if it rains put a baggie over it. Zumo are great, but way overpriced.
They are more expensive because they are made for a motorcycle. Besides being COMPLETELY water proof, they are SHOCKPROOF. I accidently unlatched mine while moving the XM line that was under my latch at 60 mph and was fortunate enough to catch it with my left hand, there was an intersection with a stop sign coming up. It didnt take me long to figure out that I would soon need my left hand for the clutch, so I tried to set the Zumo in my lap, it fell off and bounced over and over again down the road. Went back and picked it up and set it in the cradle and it worked just fine! I dont think a Nuvi (which my wife has in her Jeep) would handle that abuse. Of course most Goldwing riders arent as stupid as I!
There is another advantage the Zumo has over the Nuvi, the numbers and letter are much bigger so that you can utilize then with gloves on...even winter gloves, with out pressing two letters or numbers. I would spend the extra dough for the Zumo (of course I would, because I did back when the Zumo 550 was $850) I would not want to have to stop on a trip on the interstate in the winter, in the rain with gloves on to find my baggie and put over my GPS...but thats just me.