When I was first introduced to riding as a child, I rode in jeans for a year or two until my parents could buy me breeches (a helmet, paddock boots, and half chaps had set us back).
Nowadays, my only jeans are fairly nice ones (for fashion, not for work) that I wouldn’t want to “ruin” at the barn, so I wouldn’t ride in them anyhow, but for a while I did ranch events and rode in work jeans almost every day. It felt odd at first, but the way a western saddle’s built so thickly combined with those long western stirrups made me okay with it eventually.
I’ve never liked riding in jeans when I’m in an English saddle, though, just because jeans are so inflexible and bulky when compared to breeches, and I prefer a closer contact.
TL;DR I prefer jeans in a Western saddle, but breeches otherwise.