visually (for observing that is) there is no diference i am told. the only reason you would want to get bk7 is if you live where the temperatures are really cold... like the snowy mountains. I get temps during winter in campbelltown that can go as low as -3 and plate glass is fine. the green tinge is in the glass its self and because you dont actually look thru the glass but on the the mirrored surface the greeness has no effect on viewing what so ever.
hope this helped