A good full face helmet should not cut down on your perephrial vision at all...and I just so happen to have a copy of David Hough's "Proficient Motorcycling" sitting next to me. Check out the Risk! section. He's got a breakdown of impact locations and how likely you are to hit certain parts of your helmet on the ground in a crash...the chinbars areas are about twice as likely to hit than any other part of the helmet.
Aah, here we are!
Based on that...I'd sorta like to have a chin bar as I am rather fond of my chin...
And personally I'm not a big fan of the Snell M2000 testing (I'm along side the "too hard helmet" brigade, check Motor Cyclist magazine's
helmet test article.). I think they force manufacturers to make helmets too hard which can end up hurting you in a wreck of some sort. But I have great faith in Arai!
No matter what lid you decide on though,I highly reccomend getting full face and make sure you get one you won't mind wearing for long rides. Not gonna do you much good if you're so focused on how unfomfortable your helmet is while you're riding that you forget to...ride!