It depends on the strain for me, and what I'm in the mood for. And there are sub-types of each strain. And yes, most of the great strains are a careful mixture of the two. Someone above mentioned Trainwreck--that's supposed to be a double whammy of indica AND sativa!

SATIVA:
type #1: Racy paranoid high, sometimes stupid and giggly, bordering on psychadelic (these can be super fun if you can handle the paranoia--adventure weed). Think haze and landrace sativas.

type #2: Soaring, thought provoking high with a good body element. No paranoia--it shuts the world out while you take it in. Mildy psychadelic but it's much more relaxing. Sour Diesel is this type of high, and it's fantastic let me tell you

INDICA:
type 1#: Debilitating couch-lock, heavy body-numbing stone. I define couch-lock as not being able to do anything, and not caring to try. Want to play a computer game? Have fun staring at the screen for 10 minutes. Want food? You'll spend what seems like hours wandering around your kitchen. If it's night and you smoke this you'll be sleeping like a baby very soon. Hashplant and G-13 are two types of this strain.

type #2: Euphoric, swimming body stone, still some couch-lock but not as heavy. More of a fun social stone where you're not so spacey. Great for watching TV and playing video games and other daytime activities that require brain function

These descriptions sure aren't exhaustive as to the types of weed out there. Anyone who has the luck to be able to pick or grow strains will tell you there are hundreds of types and subtypes of highs. Some are desirable and bred thus, some are not and therefore aren't smoked by as many people.

If anyone wants to add to my post feel free!