I've owned a 60CSx since July...
Well, I can attest to the accuracy and speed of the 60CSx. It locks on anywhere from 5 - 45 seconds depending when I last had it on and how far I've travelled from when it was turned off. Indoors and under really dense tree foilage is downright freaky: it will lock in in less than 45 seconds usually and keep to under 50ft accuracy in the worst conditions. Under ideal conditions it is usually around 12ft accuracy. It's a major improvement in reception over the 60CS, and that was pretty good to begin with!
As with any Garmin, basemap is useless unless you are on the Interstate. I got Topo and City Navigator 8. I wish you could overlay them instead of switching between them, but it works regardless.
There aren't very many differences at all between the 60CSx and the 76CSx. You can easily upgrade the memory in the 60CSx. I got the 60CSx becuase I liked the smaller size and the button placement over the 76CSx. Also, the 60CSx has a $50 rebate right now until the end of the year. I haven't tried to see if the 60CSx floats or sinks (I've heard it doesn't sink very fast though). Both are equally as waterproof, and about the best in the industry, so... whatever floats your boat... :D