There are several factors to considering when you pick and choose your next strain to grow. Strains all have unique physical characteristics and chemical compositions that set them apart from other strains and crosses. The difference between a strain and a cross is that strains have been selectively bred for generations (usually 5) in order to stabilize the genetics so that the seeds usually have uniform results. Crosses on the otherhand are usually first or second generation seeds and posses a high variance in phenotypes (physical and chemical features capable of being measured). More important than picking an indica or sativa strain is the terpene profile which results in different psychological effects when smoking. THC is not the only cannabinoid that has psychoactive effects and different strains can be preferred by different individuals. In this video, I list the 7 most important things I think of when selecting a new strain to grow.
I know a lot of people might think this list is a bit excessive, but think about it this way, if you spend 4 months growing something from seed to smokeable bud, the least you could do is spend 4 hours researching your strains before you purchase.