I’ve stayed at both the Ren St Pancras and the Great Northern. While I prefer the former, I think each has its pluses and minuses.
The Ren has imposing public spaces and a hot bar. It gets very noisy and can be impersonal. I’ve only stayed in the Barlow wing, as I am noise-sensitive when trying to sleep and have heard that Chambers rooms can be noisy. YMMV. I like the lounge very much.
The rooms at GNH are all on the small side, Though I’ve seen all categories, I’ve only stayed in the top-category rooms. I enjoy their shapes, the soundproofing is terrific, I like the pantries, and I prefer breakfast in their restaurant to breakfast at the Ren Lounge, but my biggest beef with them is the lack of drawer/shelf storage space in the room, and in some rooms there's not even a good spot to stow one’s suitcase.
Particularly if not traveling alone, I’d take a jr suite at the Ren.