You guys looking for helmets...
If you're doing HPDE/track days I would absolutely recommend a full face versus an open face design, and an SA rated helmet over an M due to the better impact and fire protection. It is true that full face helmets will have a little less visibility, but this is much less of an issue on track because everybody is going the same direction (well, most of the time
) and you don't need to have your head on a swivel to the same extent that a motorcycle rider needs to dodge cagers on the street. The extra protection afforded by a full face SA certainly outweighs the reduction in visibility for track use. The last helmet I bought was a SA2005 Bell M4 Sport (bought in '08). Cost about $400, and that model has a bigger viewport than many other SA helmets.
Many/most HPDE groups require SA. Don't buy an M and assume you will be allowed on track with it. Autocross groups tend to be less strict and will "usually" let you run with an M, but check the rules of the group you plan to run with. SA helmets are good for 10 years, i.e. an SA2005 helmet expires in 2015. The new stickers come out on 5 year intervals. If you're buying today, a SA2010 helmet will be good through 2020. Any closeout deal you find on old SA2005 helmets would have to be mighty goddam sweet to justify the purchase, because it will only be good for 2 years.
ALWAYS keep your visor down on track, no matter how hot it is that day. Lots of people get complacent with this. I was coming down the straight at Summit Main and got a good "thump" on the side of my helmet, saw absolutely nothing coming. After the session I found a silver dollar-sized chunk of tire rubber between my seat and the door sill (kicked up from the car in front of me) and a black rubber streak on my helmet about 1.5" away from the viewport. If you ever find yourself considering running visor-up, think about what that chunk of rubber will do to your eyeball at 100+ mph.