I have never had an issue as a Diamond getting lounge access at the Paddington or the Metropole however the Metropole Lounge is like a zoo and becomes distinctly unpleasant at breakfast and in the evening. In contrast the Paddington lounge is much nicer with a better layout.
I would opt for the Paddington, I also prefer the Paddington room, I have never had one of the smallest room types at the Paddington in multiple stays.
Service Levels at the Paddington are not the greatest (although the lounge staff are very good) but the the Metropole is unlikely to win any awards either.