Having stayed in both I would pick the Ren St. Pancreas over the GNH any time. The GNH has small rooms and is just "odd". Plus even without the upgrade the lounge at the Ren is orders of magnitude better than anything at the GNH.
Ended up having a good stay here and would happily return (even though I'd still like to try the GNH at some point)
- Convenient lift inside St. Pancras up to the second floor to go in via the restaurant entrance (didn't realize this initially, walked around outside and in the front...oh well)
- Lobby was a total mess, some TV/media event, just jammed with people at all times
- Upgrade to a Jr. Suite cleared as noted - definitely some street noise but not bad
- Arrived at 12pm, room not ready, said 15 min, parked me in the Club for a bit which was fine. Come 12:50 or so, back to the front desk, room wasn't ready but they found me an equivalent room so all good
- Nice spread at the Club for breakfast, great attendants
- As noted elsewhere, location is unbeatable between the int'l / domestic trains as well as different Tube lines