121.5 mhz
The biggest issue from what I've read is that there are fewer assets monitoring 121.5 mhz anymore. My old AWACS does it as a routine as we'd transit across the country, but not sure how many other platforms do that regularly.
In the big scheme of things therefore it comes down to how long will it take for someone to pick on 121.5 vs. the newer 406 freq if you go down in a remote location, etc...
Doug Lomheim
RV-9A Mazda 13B FWF