As far as seat confort and space goes, there is not really much difference between the two.

In general I think that LH seats are perhaps 1/2 inch narrower but just as (un) comfortable. From memory AC flies A330-300 from YUL to CDG, whereas LH flies A340-300 BOS-FRA.

Legroom is pretty much the same (depending on the aircraft there are always 'better' seats).

LH has headrest 'wings' which AC does not have, which make sleeping much more comfortable.

Neither carrier has personal IFE, so forget that.

Food is better on LH (by miles).

On balance, I would probably marginally favour LH for comfort. Probably the biggest reason to go LH is you can have your luggage checked straight through to CDG without any immigration formalities at FRA. Also, if you miss your BOS-YUL flight for any reason you'll be stuck there for the night. FRA-CDG is served almost every hour.

Good luck