I am assuming that when you say stretchy pants - you mean the spandex material - the kind of pants you get from lululemon. So being said that, I am with sweatpants, only because I feel so awkward and uncomfortable in the "stretchy pants" but they are actually pretty good for work-out. I have a couple lululemon stretchy pants and they are kept to the "gym use" only. The sweatpants however are really comfy and easy since I don't have to worry about them clinging on to my ass. Plus american eagle is an awesome store for some fancy ones.
In any way, point being taken, I going with the sweatpants.
And in case, this question was meant to be for guys, I'd just say its about preference and what you look good in. Both are good depending on your comfort level.
I personally prefer sweatpants. I feel awkward in stretchy pants because of how they cling. The women in my family are "bottom heavy," and I feel like I'm putting everything out there for the world to see when I wear stretchy pants. Plus, sweats are really comfortable.
But there's nothing wrong with either. If you feel comfortable in stretchy pants AND sweats, just wear whichever you feel like wearing at the time.