I'd go with the PDX-LHR flight:
* Shorter positioning flight in Y
* Flying to PDX will probably be cheaper than to YYZ
* Longer flight in J, so more time to enjoy the product
* No need to deal with immigration in PDX. In YYZ, you may need to clear Canada immigration and customs especially if you will check bags.
I don't consider the DL 767 J seat to be significantly worse than AC 787 J, especially given the advantages above.