There are an equal number of pros and cons for both types of helmets. To be quite honest, one os no more safer than the other. I have a leather at one dept and I have a composite at the other. I prefer leather for the tradition and history.
I agree with Shareef. On one dept I have leather and the other I have composite. I like the tradition of leather and that the leather curls when heated versus melting with composite. I do like that composite is lighter than the leather so there's pros and cons to both. I prefer leather however.
Try them on and see how they fit and sit on your head. They are all different. I have worn Cairns 1000, 1010, 600 and 880 composites. All sit slightly differently. I have also worn a N6A for almost 14 years now. I have loved and beat this helmet to death. I am now a huge fan of the Phenix helmets. I will be getting a TL2 to replace the N6A. It is almost a pound lighter and almost 60% of the cost.
I personally think that leather is better. Its like a pair of shoes, the more you wear it the more it shapes to your head. I wear one at work and get into more trouble about it because it has shaped to my head so well that I dont ever wear the chin strap. Composite is a third the price though so you have to make that choice.
I wore a Cairns metro for a while, then a cairs 1010 then a bullard UST (or w/e its called) The leather so far is the most comfortable for me. Compared to my Bullard, the weight isnt even noticeable on my head.
ive have a n5a right now a love it, it fits good and doesnt get heavy, ive had a ton of composite helmets from the traditional "piss pot" to a1010 and bullard firedome cx, to name a few, and like mentioned befor the composite helmets do melt and leathers just worp and bend in a hot fire, i pulled the rachet out of mine when i first got it, they rub to much, with the ear flaps folded in with no mask on it fits good and with the flaps down and a mask on its the same , but the wieght it almost the same to me ,