Waypoint averaging

The accuracy of any GPS is not due to its compass, I wouldn't rely on the accuracy (in feet) figure too much, this is a calculated figure as the GPS doesn't know exactly where it is, the GPSMap60CSx only works on the original (USA) GPS satellites, whereas the GPSMap66ST works on GPS, GALILEO and Glonass so the 66 is always going to be more accurate, it does have Waypoint averaging so the 66 is definitely the one to use, eg :-

1 Select Waypoint Manager.
2 Select a waypoint.
3 Select MENU > Average Location.
4 Move to the waypoint location.
5 Select Start.
6 Follow the on-screen instructions.
7 When the confidence status bar reaches 100%, select Save.

For best results, collect four to eight
samples for the waypoint, waiting at least 90 minutes between samples.

Altitude accuracy on all GPSs is never as accurate as Latitude / Longitude measurements, but I can't see why this is required for placing geocaches

A good way of checking a geocache location is to approach the target from 3 different directions, making a mark when you get to zero feet as directed by your GPS, obviously in an ideal world the 3 marks should in the same place, however due to inaccuracies you will probably get 3 separate points, you can then estimate the 'mid point' of these 3 points for a better 'guess' at the true location, (assuming that the owner of the geocache got the correct location in the first place)