The 777 offers bigger seats, which should likely be the determining factor for you. Probably also worth paying the extra to get an unlimited legroom bulkhead.

The only benefit of the 787 is that you will arrive a bit more refreshed due to the better cabin pressure and humidity. Also a more modern AVOD system. Not enough IMHO to make up for the loss of personal space from narrower seats with less pitch.

Also the 777 flies to HND, which is a more convenient and lower cost airport to access Tokyo.

Caveat: Air Canada will begin concerting its existing 777's to a 10-abreast configuration later this year, at which point the seats will be similar to the 787. That process will take a while to complete, so hopefully you won't be affected.