I prefer to wear jeans because I don't like clothing that is very tight on me. In the winter sometimes I wear leggings under my jeans when i have to work outside and it is very cold, but that is because the discomfort of the cold is worse than the discomfort of tight clothing. When I am in a comfortable climate, I prefer loose fitting and durable clothing.
I have not yet found a pair of leggings that fit me perfectly. They are either too baggy, too tight, too short, or too long. Furthermore, many of the leggings do not properly fit around the hips. Jeans are so much easier to find in a comfortable fit and their easier to wear, making them my choice, as far as comfort.
Honestly... They're just more comfortable! Jeans are tight and sometimes they chafe and reveal your buttcrack if you bend over. Leggings are flexible so, if you don't buy them too tight, you'll never have to worry about ripping them or accidentally mooning someone. Also, contrary to popular belief, leggings aren't warmer then jeans. They actually keep me cooler then jeans do. Overall, they're just more comfortable. I could care less what some people think about them being "provocative". If I'm wearing leggings and an oversized T-shirt that means I want to be comfortable, not have some creep stare at my ass.
You can't deny the fact that leggings are comfortable especially those made in spandex. Leggings are cheap and they are sexy to look at. Unfortunately this shallow sexist world believe women are the only ones allowed to wear leggings in public. Men are severely limited to disgusting sweaty clothes like suits.
A good pair of jeans in comfortable sure. But leggings (aka running tights) such as Under Armour or Lululemon are just amazing. They are like a second skin that does not ride up and does not chafe. Heavy people tend to do poorly as this shows a ripple in the legs and rear. Others complain about the gusset which prevents camel toe and rips. Others hate that they only look good with a g-string or thong. The reality is if you but the right clothes they will feel good (yes even with the thong). Jeans are also expectational, and frequently I will wear both, but know that what I wear next to my skin is always the leggings/tights.