Actually most of the weed we get here in the U.S. is a mix of indica and sativa. Growers have cross-bred the plants, into hundreds of different strains,to get the various desired characteristics. It's pretty rare to get 100% of one or the other although you do have a better chance of getting a pure indica that you do of getthing a pure sativa.

One of the big advantages of growing indicas, beside the potency, is that the plants themselves are usually a lot shorter than sativas. This makes them much easier to grow indoors. Its a lot easier to hide a 3-4' shrub than a 10' tall tree. I think indicas also require less time to grow as well.

The big differences in the highs, like others have said, is that sativas will give you a more energetic , happy, high while an indica will ive you a more relaxing body high. If I'm going to to something like go out with friends, go hiking, etc, I definitely want something predominately sativa. If it's night time and I just want to chill out in front of the tv an indica is nice.