My response does not fit your criteria on first hand knowledge but I am doing some research on the 305 because a relative wants one. He does bicycle road races on weekends. Many of these are approximately 100 miles. If he has not bought a 305 he knows somebody that has.
If you are doing 50 and 100 miles endurance rides such as the Western States Trail the Garmin 305 may not be the unit for you. It has a rechargeable lithium battery with a life of approximately 10 hours. On a 24-hour ride you would need to have a least two fully charged replacement batteries. A critical element in your activity is the health of you animal. If the heart rate does not pass the vet check you are out of the event. I believe a key would have a team member put a fresh battery in the unit at a checkpoint at the 6-hour point.
I am including a link to the 305 manual.
http://www8.garmin.com/manuals/984_OwnersManual.pdfI think your idea of two units is smart, the Garmin Legend HCx or the Garmin Vista HCx with high sensitivity receivers, 25-hour battery life, micro SD memory card, and the ability to use Garmin’s Topo 2008 MapSource software would be very useful.
This is a full-blown lightweight handheld, mapping gps with good battery life. You can create Proximity Waypoints with a warning tone out from the waypoint at a desired distance. You can also store tracks and routes on it. As an example you train on a course and store your track. On the day of the event you get off course you can look at the screen and see your relationship to the course and find your way back. Encountering a sow bear and cubs on a trail does put the rider off course.
A link to the Garmin Legend HCx manual:
http://www8.garmin.com/manuals/eTrexLegendHCx_OwnersManual.pdfThe 305 is not a mapping gps, and does not have a Basemap or the ability to use mapping products. The Garmin Mapping gps units have a basemap, and you can load TOPO 2008 or other MapSource products on it. The nice thing is that it will give you the ability to have trail maps of the area you are riding. Be it the Alleghenies, the Rockies, or the Serrias.