Since Aslan has a lifetime guarentee on their harnesses, you can rest assured that they are all built for longevity and if anything is damaged under "normal" conditions, you can send them in for free replacement or repair.
I'll always go for leather, no question. Because leather items are made of skin that was once part of a living thing, they need special cleaning (glycerin saddle soap, minimal water) and occasional oil replacement (leather conditioner) to keep from drying out. They also have to be protected from mold and mildew. I love caring for leather, but I understand that not everyone feels this way. Leather gear is higher maintenance compared to synthetics. If you are looking for something you can wash with regular soap and not worry about, vegan leather might be for you. If you are looking for something you can take in the pool or shower, go for vinyl (I'm actually looking into vinyl harnesses for that reason). But nothing can replace the feel of leather on skin and the way it conforms to your body over time. In my book, there is nothing better.