It looks like you have done your homework. I chose the 60C after studying the same units. They are indeed almost identical, having the same screen, the same buttons, and even the same firmware--the downloaded firmware files are identical.
The 76C(S) is bigger than the 60C(S) and weighs the same, so it floats but requires a bigger pocket. That is not a factor on the bike, but might be for hiking and skiing. It was for me.
The 76C(S) buttons are above the screen, the 60C(S) are below. In practice, that seems not to matter. Both can be operated with one hand easily.
Neither base map is useful for cycling. They show main roads and interstates, which you do not want for routing. They can give general orientation, but if you do not know whether you are north or south of I-80, you are beyond help from GPS [grin]. I'll suggest Garmin's City Select for routing on streets and, if you are in the US, US Topo for hiking or cycling where topography is significant.
The larger memory is definitely useful for really long rides. Detailed routable street maps for the entire Northern Tier route will fit into the 76C(S), but the 60C(S) will get you only a little more than half way on one load. Both, however, are limited to 1000 waypoints. The Northern Tier has 3380, so some reloading would be necessary anyway.
The 60C and 76C provide GPS altitude, which is accurate long term but noisy in the short term. Single readings are about +-50 feet (95% confidence) with good reception and about +-100 feet with poor reception. The barometric altimeter in the (S) models is much better short term, perhaps +-20 feet regardless of reception, and is calibrated against a long term average of GPS altitude to compensate for atmospheric pressure changes. I routinely make profiles of my hikes and cycle trips from the GPS altitudes, whose noise is visible in the profile before I remove it with a simple low-pass filter.
The magnetic compass in the (S) models is of little use. The unit must be held level for good readings, requires recalibration after a battery change, and reduces battery life. Judging from comments on the newsgroup, most people turn the compass off. GPS track is valid within five seconds of movement. If you want good heading data while stationary, a separate compass is better, IMO.
As for software, there are many PC programs (some free), but almost no Mac programs, that let you create waypoints and routes, then load them into the GPSR. They will accept waypoints, routes, and tracks from the GPSR, store them, and plot them on maps.
These programs cannot load maps into the GPSR. Garmin's maps are proprietary and loaded only with their MapSource programs that comes with the maps. MapSource handles waypoints, routes, and tracks as well. It works fine for planning trips.
The GPS User Guide on the Adventure Cycling web site describes several of these programs and tells how to use the ACA waypoints with them.
For completeness I should mention some hacking work that does let you create simple maps and load them without Garmin's software, but it is a work in progress and not very easy to use. A map large enough for useful cycling would be totally impractical. If this interests the geek in you, browse the newsgroup sci.geo.satellite-nav, where it is a perennial topic.
Well, there are one man's opinions.