Genotype is the genetic potential, phenotype is how it actually expresses those genes in a given environment. Chemdog is a great example, take 2 clones and grow them with different lights and media and you get very different finished products.
Generally in cannabis we use phenotype because so many f1 crosses are fairly consistant on making 2 or 3 distinct sub categories of crosses.
Example, you make a cross and you see 3 main progeny, one leaning mom, one leaning dad and one an even blend