There are not many places where you can find both sides of the argument presented. At present I thing the best bet is to read plenty of sources, and make up your own mind. The largest resource for cycle helmet research of which I’m aware is the Bicycle Helmet Research Foundation, at http://www.cyclehelmets.org – but I would say that, since I am on the editorial board.
John Franklin is probably the foremost cycle safety expert in the UK. His site. http://www.lesberries.co.uk, has many excellent resources including a fascinating analysis of the safety effects of cycle facilities.
Wikipedia is the free encyclopaedia anyone can edit. It has an article on cycle helmets: http://en.wikipedia.org/wiki/Bicycle_helmet which is pretty balanced, although it suffers form the occasional attempt to hijack by one side or the other. Wikipedia’s policies of verifiability and neutrality usually prevail, though.
My own site (linked below) has more of my thoughts as well.
CTC, the national cyclists’ organisation, has an extensive website at http://www.ctc.org.uk, which includes help and advice on cycle safety. CTC’s magazine, Cycle, which is free to members, is one of my favourite cycling publications, with excellent technical and legal input from Chris Juden and Douglas Carnall.
If you know of a good resource, do contact me and let me know.