Black birkin. I say that because I am not a kelly girl. BUT, I'd say that anyway in this case. Classic and you already have a red bag. And the red you have now is much "redder" than the vermillion which to my eye has a lot of orange in it.
Agree with everyone here. A Birkin and a Kelly are different but not THAT different if they're the same colour (unless you really really love the red colour, then you'll try to collect everything in red), otherwise go for some versatility and pick a black bag - since you would want one in any case.