This might not be news to anyone, but then it might still be useful to someone
Cannabis Rex - surprisingly good speaker (not that I thought it would be bad, it's just got a lot about it I didn't really expect), more plentiful and roundeed bass than the G12N, and a really nice 'piano' presentation to notes. Good dynamics, and quite bright but with absolutely no 'ice pick' top end.
G12N (20 watt heritage jobbie) - sounds more like a 60's Marshall's etc, with a real sense of time travel in the noises it makes. Darker than the Rex in sound, but cleaner/quicker between notes. Much dryer bottom end. Overdrives more pleasingly if you like that classic sound. More percussive than the Rex.
I've had them both in my Blues Junior, with a long enough speaker lead to have my PP-18 sat on top. The Rex was bought to either become the speaker in the cabinet I build for the PP-18, or to replace the G12 that I've had in there for a while now should I move it onto 18watt duties.
I will 100% be using the Celestion with the PP-18 as it sounds way better to me as a combination, and I would 100% recommend trying a Re in a Blues Junior as it is different, but as good as the G12N for about half the ££'s. I guess it sounds American