I am not sure if you have read this thread
http://www.wakeworld.com/MB/Discus/messages/1/468230.html?1183054752 one of the major concerns seems to be having a helmet that fits you properly. I am not sure that a one size fits all helmet can provide the same protection that a helmet built for a specific size can.
If you can only afford to spend that much, i would tend to say an adjustable helmet is better than no helmet.