Well the satvia may take abit longer to flower but will be a nice smoke,,,,
when it comes to cointainer size bigger is normally better but there are pros and cons,,,as vtm said, you can grow a big plant in a 3gallon cointainer, 11litres where i am, or if you want a bush go up to 5gallon,18litres, but with the smaller container you have to water more often and may get abit root bound( where the roots can't expand any more and wrap around the container,) you see this in the leaves when it happens, it also mean if you were to transplant the plant will get more plant shock, but then again it can be just what the plant needs,
well at eight inches you should be in a 3gallon(11litres) and also as its a sativa when its between eight and 12inches tall it should be ready to flower as it will stretch in flower to a good 3footer,,roughly,, and going off what i have grown,,