I have both a nuvi 650 and a 76csx.
I started out with the nuvi as I had bought it for my wife and was able to borrow it now and then.
paperless geocaching is pretty nice for the nuvi, but I did run into problems with the short battery life of about 4 hours or so a few times.
also, the screen on the nuvi is pretty fragile and is extremely easy to scratch up. this and the water proof issue can be handled by using a case such as one made by RAM, so it leaves battery life and awkward size being the only real issue when compared to a 76csx.
it's nice being able to use it for paperless geocaching, but I found switching between the various screens I needed to be more of pain than just using my palm for the paperless aspect and the gps for the gps aspect.
I also use the 76csx as my driving gps leaving the nuvi for my wife. it doesn't talk to you, but beeps where there would be voice prompts. I find this perfectly acceptable. also the screen size is about half of what you get with the nuvi but this can be solved by placement of the gps by mounting it closer to your eyeballs. I mount my 76csx right next to the steering wheel whereas we tended to mount the nuvi significantly further forward on the dash by the windshield. another way to solve this is by using the RAM magnifier which does a very good job of enlarging the screen. unfortunately there is too much light in the cab of my truck which creates a lot of glare, but it works fine in my car.
I have also found the compass of the 76csx more useful that I thought I would. it is also a nicely rugged device.
having used both, and if I was just buying one gps for myself, I would go with the 76csx.
my wife however would insist on a gps with turn by turn directions that include the street names in the voice prompts, but she's not a geocacher.
if you are buying a gps to mostly give you voice directions and also maybe use it for geocaching now and then, the nuvi is great. if you want to split 50/50 or mostly use it for geocaching and are happy with beeps instead of voice prompts, I would go with the 76csx.
to get turn by turn directions on the 76csx you need to buy an additional map which will add to the cost. also the 76csx doesn't come with a auto power adapter or car mount, so be sure to takes those costs into account when making your decision. also a keep in mind a water proof case for the nuvi if you go that route.
if you want voice prompts and want to geocache regularly, I would recommend buying both a nuvi and a lower end high sensitivity gpsr, as there isn't a perfect single solution to meet those requirements.
I guess the other aspect would be a park and grab geocacher, the nuvi would work perfectly well for that. but I find myself doing a lot of hiking or day long geocache runs where I will be away from a car, so I need the longer battery life and the ability to swap batteries of a trail type gps.
for example the last cache run I did was for a puzzle cache which was composed of 7 caches scattered in a local historic park. I needed about 6 hours of battery life so I couldn't have done this with a nuvi without plotting out the caches ahead of time and only turning on the gps when I was near the caches, which I don't like to do.