I'm thinking to drop few lines about an experience i made last winter regarding those two plants.
Is hard to fine accurate comparision between two clones because one never knows how they were spaced and especially how much their rootstock/nutrition can vary.
Well, in my case, i guess, i have kept as close as possible two varieties in rigorously the same environment.
I grated both valentine and coctail on same rootstock, at the same height in the same spot, so everithing they got was rigorously equal. Just... i'm not sure what rootstock it is since i reused a failed clementine which i'm not sure about the origin, but i guess it must be citrumelo.
They grew nicely during summer then winter hit.
In the first column you have the average temperature in celsius and in the second the minimum temperature in celsius. As you can see we got till 17F with about two weeks of sub freezing nights (and i mean, not just light frosts).
During all this time both the valentine and the cocktail were outside with just a light frost cloth.
Well somehow they manage to survive with just little dieback and not becoming completely defoliated. No flowering this year but they bounced back.
Here a picture of them just after the frost at the end of january (the right part of the plant is cocktail and the left part is valentine, frost cloth moved up to show leaf damage):
They managed to survive! Pretty beaten but alive. Of course some degree of defoliation was to be expected and so happened (BTW you can see on right bottom corner of the pic a twig of ilya11's voss bloomsweet grapefruit completely unfazed by the cold)
This is them a month later:
The cocktail grapefruit got beaten a little harden than Valentine, even if i have to admit that on a small sized plant like this one is hard to make conclusive statements.
What can be said is that Valentine and Cocktail when dormant are pretty cold tolerant. Also im not completely sure but i suspect that keeping some leaves alive on more exposed twigs may be useful to help the plant in avoiding entire twig dieback. It's just a thought though.
Right now coctail has also a flower while valentine is a little more shy, but they completely recovered.