I prefer to stand, but in practice I often have to sit, as my back and heel (plantar fasciitis) start to hurt after an hour or so of standing. Standing in one spot hurts my back; walking too much hurts my heel. One solution is to bring a chair and sit in it for frequent breaks. That helps some.
Ideally I’d start a painting standing, then sit for a while, then stand for a while, etc. But doing so obviously changes one’s point of view! It might work for a landscape with a big vista, but it certainly won’t work for a portrait or still life.