they are cheaper when they are smaller,and i too would rather buy a baby fish than a bigger older one. i have had those days at the lfs when you see a fish thats a little too big for most people's tanks and you take him home, its almost like an aquarist weekness.
I like the idea of getting a fully grown fish as it does take out all that waiting. But watchin them grow, and their colors brightening.....thats the best...