The DL 767 business class seats are very tight. I flew JFK-MAD on that product last year. The service was pretty good, but even as a fairly small person I felt cramped.
The AC 787 looks like a very nice product to me. If I had to choose that's what I would pick.
You shouldn't need to clear immigration and enter Canada on the outbound. There's a transfer checkpoint, so especially if you are connecting AC-AC you should be fine. I flew DCA-YYZ-HND last spring and it was very easy. No real line at the transfer checkpoint. Still a lot of walking in general though. I'd rate the Maple Leaf Lounge at YYZ ahead of the DL club at JFK as well.
The system on the return is a little more convoluted since you will clear US immigration and customs at YYZ. But I think you will also need to enter Canada first so make sure there's plenty of time on that connection unless you have Global Entry.
As for using the voucher, you can see if the AC flights are UA codeshares in which case I'd imagine that it might be possible. That's probably a good question for the UA forum