Marans are tasty, they're on my list as a dual purpose meat bird to work with in the future. But I personally would go with the cuckoos rather than the black coppers -- the BCMs are awfully expensive right now. The egg color on the cuckoos is lighter, on average, but good quality cuckoo marans eggs are still plenty dark (as dark or better than welsummers). They're also easier to find, especially if you're not particular about specific pedigrees or leg feathering. Unfortunately hatchery cuckoo marans seem to vary wildly from excellent to absolutely terrible, even within the same hatchery, so I'd suggest finding an actual breeder (but I guess that is true of any breed).
I agree that the welsummers would hardly be worth the feed you put into them. I'm not that big on DP breeds either, however when choosing one to cross with my CX's I did go with a BCM, and made my choice after weighing a pen full of full grown DP roos of many breeds and all of breeder quality. SO in my humble opinion of your two choices offered BCM is the hands down winner, and the really dark eggs are very cool!
Sorry I'm responding to this post after it's a couple months old, but I just wanted to mention we just ate a couple of our welsummer males that were about 5.5 months old. I don't feed our birds much (they free range and I give them some scratch) so it was worth the cost to me. They were super-tasty as chicken nuggets for the kids. I ground the meat up in the food processor. There was less than 1 lb per bird of breast meat, but organic breast meat at the grocery store is currently $9/lb. Then you also have the legs to eat. I only got a little off the wings. A lot of the time I don't even de-feather the bird before cutting off the meat. I just pull off the skin in front and cut off the breast meat, and break the hip-bone backward and cut that off too after pulling the skin off. It doesn't take much time at all that way to process them.
The welsummers we just had were more meaty than the buff-orpington rooster we had this week also. He looked huge, but it was mostly feathers! So I guess if you free-range the birds, the wellies might be better than some others as far as finding food for themselves. We've only had chickens for a year though and I haven't done meat-birds, so I don't have much to compare to. I love the welsummer egg-color too (some spotted eggs).