Both cover 10x8 well, (a 240mm Symmar S covers 10x8).

If you're keeping the 5x4 camera the 300mm should be useable with it, the 360mm might be too long (depends what camera you have). I bought a 300mm f9 Nikon M to use on my 5x4 while planning to move up and also shoot 10x8 - which I did quite a few years later.

Other than that the choice is personal, what would you use most.