Breeches, cotton or wool. I get them from What Price Glory. They have multiple different types.
Denim is hard on leather. It's a "hard" fabric that wear quite well but is harder than the leather it works against. That means instead of wearing out a $50 pair of jeans you're wearing out a $1000 saddle. Not the best choice, eh?!?!?!?!
In the old days no self respecting cowboy would have been caught dead in denim. It was the fabric of miners, puddlers, and convicts. No military riding breeches were ever made from denim (although some working "fatigue" uniforms did use it, as did the old and now obsolete Navy dungarees). Civilian riding pants of all types were made from cotton, linen, wool, etc. No one ever used denim.
If you're riding a synthetic saddle then maybe it doesn't matter. If you're riding a leather saddle it does.
Breeches, definitely! I shudder when I think of how hard it is to bend my knees for mounting in jeans as well as what they do to my saddles! I love, love, love my cheapo Riding Sport low rise side zips!
DD rides in jeans and her less than 1 year old saddle looks like crap already, whereas my saddle fitter complimented me on how fantastic my 6 year old used saddle still looks after my having it for 2 years.
I did splurge on a a few pair of jean breeches, and have decided the fabric seems to be a little too stiff/hard for my dressage saddle, but I do use them for jumping.
Boy Answer: Both. Saddle's already scuffed up top, so no savings there... have breeches, tend to prefer them, but usually ride before and after getting other work done, so jeans are pretty common as well. Probably 70-30 jeans to breeches. If its HOT (90+), the breeches ratio goes up because I'm wearing my kilt when I'm not riding.