I think the choice of ship has a lot to do with any experience in 'The Grills'.
First you have to decide if it's QG or PG. When folk say they travel in the Grills, they often don't specify which one so it can be hard to get the correct impression.
I haven't seen any cabins on QM2 but I have seen the QG dining room and I wasn't impressed but then I'm not a fan of that ship. I have heard however, the cabins are very spacious and that includes PG.
On the Vistas, the PG cabins aren't that much more spacious than Britannia but they have a huge added bonus.
Decks 11 and 12.
These two decks are deemed 'The Grills'. Deck 11 has both restaurants and having taken afternoon tea in PG, the restaurant is almost a t2win of QG. Bright and spacious. The lounge is there, not huge and can get very busy at certain times[ usually pre lunch and pre dinner but a lovely place for your complimentary teas and coffees when the bar is staffed. The concierge has their desk there too.
Outside you have a sun deck and between 11 and 12, i have never yet not managed to get a sunbed or chair if needed.
Then we have QG. The main differences being the cabins and the choice of food in the restaurant. We love the restaurant but it's the spacious cabins which draw us time and time again, The Butler is a nice touch and is always at the end of a pager when on duty for anything you need but one can survive without that. Same for a 'free' bar set up.
Is QG worth the extra over PG? A very personal decision just as is PG worth the extra over BC etc.
We have gone from a Q5 over the years to a Q2 and a Q1 purely because we want the space. It's up to the individual to decide if PG/QG is right for them.