it does not matter that much. only thing of importance here is that the leather will be still usable whereas the synthetics are "one crash only" type of thing. leather does not breathe either. i am not arguing for one or the other, that is an endless debate, i am merely saying there is no "best". each has its positives and negatives. if i would be riding a sport bike on a track or doing high speed freeway riding, leather is a no-brainer. anything else, synthetics will be more user friendly. also, understand the difference between nylon and polyester. one is tough and rough(ie. cordura), the other is soft and weak. most cheap product use polyester that provides very little protection so they layer it whereas nylon(like cordura) will be much safer choice.
PS: if you crash, for the love of anything holy, do not slide but roll(ie. be active during the event, not passive). it is better to get bruises rather than getting your skin peeled off.
the leather will be still usable whereas the synthetics are "one crash only" type of thing.
Leather may still be usable. Also, one of the primary failure points is the stitching, which is synthetic. It can especially be a problem if the gear is older and dust has abraded the stitching for years, particularly if leather conditioner was used that is oily and attracts dust.
Synthetic can be repaired, which is why it's often worth it to go with a more expensive brand. I've had my Aerostich Roadcrafter for 25 years and it's served me well, though it aged out of their servicing policy a decade ago.
PS: if you crash, for the love of anything holy, do not slide but roll(ie. be active during the event, not passive). it is better to get bruises rather than getting your skin peeled off.