Բանաստեղծություն [banasteghtsutyun] has much deeper meaning, just like the root of the word, բան [ban], does. so, բանաստեղծ [banasteghts] is not just a poet, but also, a worker, philosopher, sculpter, etc...
all in all, it has possitive meaning, meaning, it is used to describe something (or something) worthwhile, valuable, of high class. just like բանաստեղծություն [banasteghtsutyun] is used to descibe a good, valueable poem, in contrast with ոտանավոր [votanavor], which is used to descibe any rymic saying (such as ծիտիկ, ծիտիկ մոտ արի, քեզ տամ ցորեն ու գարի).
btw, it armenian literature we also have a unique genre called, արձակ բանաստեղծություն [ardzak banasteghtsutyun].
as far as քերթել, քերթող [qertel, qertogh] is conserned, and this is just a guess and how i personally perceive the word, it has more scientific connotation, while բանաստեղծել, բանաստեղծ [banasteghtsel, banasteghts] has more artistic meaning. that is, for one to քերթել [qertel], or be called քերթող [qertogh] he has to have knowledge, has to have done reseach in the topic. as such, we have a քերթողահայր [qertoghahayr] Մովսես Խորենացի [Movses Xorenatsi], the first քերթող [qertogh]. note, not Մեսրոպ Մաշտոց [Mesrop Mashtots], who i think is the first բանաստեղծ [banasteghts].
and as Arpa mentioned, note mix it with կերտել [kertel], which i suspect have some connection.