My main advice is if you want to eat some Jersey Giants - Make sure you get stock that you know from some sort of praise to be of good enough size and weight. My last JG's I got were pretty much typical chicken size, I was pretty disappointed.
After that, I'd say they will make good dual purpose but not good meat birds. By the Standard of Perfection, a young bird at just around 7 months old should exceed the weight of almost all other breeds out there, which is great, but as a meat bird most people would want to see that weight reached earlier. Also, the question is, how much is bone and how much is meat? I always hear Jersey Giants grow, THEN fill out.
All said and done, I agree with the previous person's comment on Light Sussex. Any imported / Greenfire stock of Sussex are faster growers and very well filled out birds. Very big birds, too, and if you're in such climate - They're very very cold hardy.
But as meat birds I'd always better recommend a different breed or a hybrid of sorts. Dual Purpose though - Go right ahead.