I second that Delawares can be difficult to keep in a mixed flock. That was my experience as well. I had to have my "Delaware" flock and my everything else flock. BUT, if you're only looking for one dual-purpose breed, I feel strongly that the Delaware is probably the best option on your list. They are especially good egg layers at this point in their evolution, and certain lines and hatcheries also produce good meat birds that develop out around 24ish weeks or so, which is standard for a heritage breed.
That being said, my next birds will be Houdans and Russian Orloffs based strictly on aesthetics and their fascinating histories. I LOVED my Orloffs and can't wait to get more.