My experience with the Champagnes I raise (since 2022) is that they are all over the place in terms of temperament, growth rates and adult size. I've had some bunnies with tremendous growth rates (I had to show a buck who was still a Junior in the Senior class because he weighed over 10 pounds at 19 weeks), while in the same litter I had a buck who never did make senior weight. My does have given me anything from 4-8 kits per litter (although, I've had feed problems in the last year, so that may be smaller litter sizes than would be normal). They've all been very good mothers. I have had some problems with sore hocks, which were solved in the short term by giving them resting boards. I intend to breed that out of them, though, as well as the temperament and growth variations, like I have with the Satins.
There is a pretty small population of most breeds here in AK, and like
@olhippityhop says, temperament - and all other traits - will vary by the individual and especially by the genetic lineage. You can certainly breed for temperament; any rabbit that bites or even lunges tends to get pulled from my breeding lineup, and as a result I never have to worry about my Satins now. The Champagnes in my barn are a work in progress.
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As far as weight goes, the standards for Champagnes and SF are almost identical. Ideal senior weights for Champagnes are 10lbs for bucks, 10.5lbs for does; SF are 9.5lbs for bucks, 10.5lbs for does.
I don't raise Silver Fox myself because I prefer sleek rabbits.
Friends of mine who do raise SF say similar things about them - some are big, some are small, some are nice, some are not - except that overall meat type is much better in the Champagnes. SFs up here are all pretty flat.
It's best, if possible, to talk with the breeder of rabbits you're considering, to find out what they expect from their herd. You might find yourself drawn to one breed over another simply because you find some excellent, friendly, helpful people raising that particular breed in your area.