I can only agree with most of the previous comments. 121.5 would be low on my list of priorities. If Valley was operational, that would be my first choice - otherwise I would go with Caernarfon, as the nearest operational ATC service. Even then, if I had only 1 comm box, these would get dropped for some company frequency for some of the time.
The reason that it would be low priority is that I woud not expect any transmissions I made on it to be received by a ground station, due to the terrain. I would expect to get more useful information about other traffic either from Valley (or even from Caernarfon).
In the hills, you would not expect reliable comms on any VHF frequency anyway, so see and avoid would be the watchword.