In my experience, almost all screens tend to be at least reasonably good and the boost in contrast is good, but can be offset by the decline in lumens and must actually be taken into consideration against what type of viewing material you will be doing with ambient light.
That is you have several situations:
1. You have a light controlled room 100% of the time - go with a white minimal gain screen (Carada Brilliant White)
2. You have no/limited light control and will be viewing movies almost exclusively in that room under less than favorable conditions - go with a grey screen, even better would be a positive gain grey screen (Firehawk from Stewart for example)
3. You have no/limited light control and will generally watch movies after dark only (critical viewing) but will mostly use it for sports during the day when you have no light control. - Go with a minimal gain white screen typically
Option 3 I think is what many of the people with ambient light intend to do as has been the case with all of my client's in non-controlled rooms. They wanted to watch sports during the day in HD, and movies after dark. To maximize the 'POP' I typically went with a 1.3-1.4 gain screen in white for those people and they have all been very happy. I have used grey screens in other rooms and have been seriously irritated by the lack of pop for sports when the lights were on, but no more impressed by movies when the lights were off.
I consider minimal gain white screens to be one of the best options out there and suitable for 90% of situations.
If you must go with a grey screen, then I would be lookng to get a brighter projector if at all possible. The downside is that once again, if you go with a brighter projector, you may be giving up shadow detail and contrast ratios.
It's a mixed bag, but I am a big fan of the white screens.
