Definitely outdoors. In the driveway, (at night), at the pool, in a lake. All places over the years. Oh yes. Sun, sky, blankets, nothing better. Indoors if you want to sleep or need a shower.
Indoor usually. I like to save outdoor for special occasions. It does feel great outside, but you wouldn't want to get used to it, it would loose it's fun eventually.
I think i generally prefer indoors for comfort factor, however had some AMAZING hot al fresco sex last weekend. http://hotmandy.wordpress.com/2009/03/28/riding-dirty/ I do love fucking outside... the thought one might get caught is really hot plus its great to be a bit of a hippy and have a bit of nature appreciation. Kind of makes one feel very liberated and free x
Indoo is nice, romantic and comfortable. Outdoor sex is exciting, a bit nasty and most of the time you are go for a quicky before anybody discover what you were doing.