I'm just posting in regards to materials....
When it comes to hardware, solid brass and stainless steel are the best choices. The "nickle plated" that snaps is nickle plated zinc, not nickle plated in general. You can get nickle plated solid brass for instance and that's a great piece of hardware for those that want silver. But zinc makes me cringe... I have some personal horror stories with it as well as all the ones you hear about online and such. I recently started making dog things to sell and spent days trying to just source materials bc I am dead set on only offering the best quality materials I can. I learned there is more zinc out there than I even want to think about and less availability on the good quality items. It will cost you more to avoid zinc hardware though(it's cheap and easy so there's a lot of it).
On the leather vs biothane: biothane is going to be basically a zero maintenance material. You can take it through more or less anything and only have to give it a rinse through water afterwards... But it will never break-in and mold to your hand like leather will. They're both great materials... but I tend to like leather for everyday comfortand biothane for doing "ewww" things. I'd say think about what is more important to you.
When it comes to hardware, solid brass and stainless steel are the best choices. The "nickle plated" that snaps is nickle plated zinc, not nickle plated in general. You can get nickle plated solid brass for instance and that's a great piece of hardware for those that want silver. But zinc makes me cringe... I have some personal horror stories with it as well as all the ones you hear about online and such. I recently started making dog things to sell and spent days trying to just source materials bc I am dead set on only offering the best quality materials I can. I learned there is more zinc out there than I even want to think about and less availability on the good quality items. It will cost you more to avoid zinc hardware though(it's cheap and easy so there's a lot of it).
On the leather vs biothane: biothane is going to be basically a zero maintenance material. You can take it through more or less anything and only have to give it a rinse through water afterwards... But it will never break-in and mold to your hand like leather will. They're both great materials... but I tend to like leather for everyday comfortand biothane for doing "ewww" things. I'd say think about what is more important to you.