I personally like AC in J. The seat is definitely a plus, unless your UA flight would be in Polaris class (764 wouldn't be). Food is good, soft product very decent. Regional jet not a big problem in J (seat comfortable compared to EU J), even if it is a tad of a long run in your case (would suggest the side with only one seat). The main downside: YYZ on the return trip (if you checked bags, you will need to wait in a transit lounge for customs to clear them).
I would consider LH and UA equal on many aspects, with an advantage to LH on the soft product (more predictable).
Ultimately, my decision would be based on the cost differential (if significant) and also schedule. If you are being asked to spend significant time in EWR (not the most pleasant of airports right now) then I would try something else. Arrival time and total transit time would matter too.