The Snell Rating specifies a minimum level of impact protection. It doesn't have a big influence on the exact materials, overall quality and comfort. I have owned 4 or 5 helmets over the years, and tried on probably 15 or 20. The biggest difference I could see in pricing was comfort and fit, followed by bells and whistles (graphics, prewired for audio, etc.).
SCCA may actually outlaw full face shield helmets, so you should probably wait a little longer before you buy. SCCA is doing an investigation into injury risks due to full face helmets and potential jaw and neck injuries if the airbag should inflate. They are concerned about the closer proximity and additional leverage that the airbag impact might have when contacting the lower face/chin shield.
I own 2 Snell full face helmets, a Bieffe and an HJC, but it looks like I will be buying my daughter and I new open face helmets for the spring season.
The official announcement is here -
http://www.scca.org/_Filelibrary/File/AirbagAdvisory.pdf
Participant Bulletin
DATE: November 16, 2004
FROM: National Staff
TO: All Participants
SUBJECT: Airbag Advisory
It has been brought to the attention of SCCA Technical Services that the use
of full-face or closed-face helmets while driving vehicles with active airbag
restraint systems may result in injuries in the event of a crash that deploys
the airbag. Because of the location of the steering wheel relative to a
driver’s position, the airbag axis is on a level with the driver’s chin. In a
crash with airbag deployment, contact with the chin area of a full-face
helmet can be so powerful “that the risk of fractures to the jaw cannot be
ruled out“ (Hubert Gramling, FIA Institute, FT3/AF, 18.5.1999). This applies
to vehicle that may be used in Solo, RallyCross, High Performance Car
Control Clinics, etc.
Therefore, it is highly recommended that full-face helmets not be used in
vehicles with functional airbag systems. Potentially more restrictive language
is currently being considered for 2005, which could appear in an early 2005
issue of FasTrack. If you have any questions, please contact the SCCA at
(800) 770-2055.