Both. I ride two sided pedals, and I often find myself clipped in on the left and riding flats on the right. I've done straight flats as well. No real difference IMO, at least for my purposes.

I prefer SPD so I can walk around off bike. I really like these pedals: https://www.rei.com/product/116154/s...000-spd-pedals. Got mine dirt cheap at a Garage Sale, but I'd definitely drop the full price (well maybe sale price) on another set. If you want flats, RaceFace Chesters are nice.