Depends-
And I say that as a registered Gelbvieh breeder.
It depends on the current makeup of your cows, what you want them to be and what your marketing plan is.
If your herd is currently full of other continental bloodlines, and you like those cattle, then I would opt for a British breed.
If your herd is basically british or british/american cross, then I would opt for a continential bull.
Then I would decide if you are going to market every calf to feedlot or if you plan to raise and or sell replacements. If you are gonna sell every calf and they will end up in the feedlot, then its hard to argue Charolais or Limousin.
If you want to keep some heifers, sell some heifers, and market some darn good continential/british steer calves then either Gelbvieh or Simmental should work. The Gelbviehs would give you more of a solid color pattern (in general) over the Simmental.
The Gelbvieh also are generally very docile and very fertile.
I think which ever breed you choose, pick a bull that will help your herd from a phenotypical and genotypical perspective.
If your herd is currently a good blend of continental and british, I would strongly look at a Registered Balancer (Gelbvieh x Angus or Red Angus).
Regards,
Smoky