Hi M55.
Think it has been discussed before, but can`t find the thread link.
Leather is nice in the hand, and looks `traditional`, will last if cared for, but is a bugger to maintain after it has been soaked, and dried a few times, and can be difficult to use in really cold weather - although no doubt some will say different...

. It can also be difficult to follow in low light, wooded / leafy terrain, due to its colour.
The new variants, such as biothane, are highly visible - fluorescent, also durable, don`t suffer from +/- temperature, but are very low mainteneance, as they are synthetic. To some they may not be aesthetic, but if you are on a track of a wounded deer, that is not really relevant.
I imagine it also depends on how frequently it will be used. Some of the UKDTR / UKSHA guys on here are using theirs day in day out, for training, and real tracks, in all types of cover / terrain / weather, so they need robust kit that is functional, will do the job, and not let them down when needed most. Biothane fits that brief.
I have both, and 99% of the time, go for the biothane... but still like the leather version for occasional summer / dry weather use.
Probably loads of different opinions from tracking people better suited that me - I am a novice at tracking.
All the best.
Neil.