STJ - sorry to say it dude, but its in the bottom right
and as for the origional question. Bottom line is the prefered setup as it allows for a Vertical regulator (looks like a fore grip, but far more useful) which increases consistency (which will in the end effect accuracy).
Also a bottom line allows for the marker to be ballanced. so its not front heavy when loaded with paint.
This alows you to kick the bottle into your shoulder, which allows for a better aim.
Thats all in theory.
Basically, get a bottom line put on, because it makes it easier to upgrade the marker in the future. And Allows for a decent reguation process before the CO2 / air reaches the internals of the marker.