I think when you talk about the safety of a flip up helmet, you need to go back to what Harry Hurt said a number of years ago,
"the cheapest full face DOT approved helmet available is much safer than the most expensive, Snell Approved, open face helmet that you can buy".
I have always thought that the difference in safety between a full face and a open face is the chin guard and there isn't any way that a flip up helmet is going to have as strong of chin guard as the one piece chin guard you get with a full face. Granted, as the flip designs improve so will their safey but they will always be a bit behind their full face counterparts.
With all that being said, I just bought a Scorpion EVO 900 helmet to go with my EVO 700 (Snell Approved) and EVO 400, which actually scored higher than the 700 in Motorcyclist's helmet tests. The 900 has a very robust chin design and appears to have a very good locking mechanism, its hard to believe that the flip would open or that the helmet could come off my head in an accident. Its about 6 ounces heavier than the 700 and most of the extra weight seems to be in the front, when not riding it seems to be "front heavy". Its also noticeably wider than the 700 and seems to have as much, if not more, padding than the 700.
The 700 is a more comfortable helmet (it has also been broken in to my head), but the 900 has some nice features, such as the inner drop down sun screen. I am thinking that the 900 is probably a better hot weather helmet but I am wondering if the extra weight is going to be fatiguing on a long day.