i have a Bell Sport-II full face helmet. i usually wear the visor up though, since its a lil easier to breathe and the visor gets foggy soemtimes. i would recommend a full face helmet because you would want something that protects your face in a crash. it also protects you from spinal injury because the front part of the helmet will hit your chest before your head is able to snap forward in a crash, in an open face helmet, your head can move much farther forward. the cool thing about the visor is that when you spin off track (and believe me you will...unless you drive like a sissy la-la), it keeps whatever dirt, dust, car parts, and crap you kick up out of ur eyes and face. i guess the only advantage of an open face is the cheaper price and more room for glasses...
<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/withstupid.gif" border="0" valign="absmiddle" alt=':withstupid:'>