They are not the same fish and do not have the same functions, they just look similar when the engineer is a juvenile. I know you're looking into a nano set up, do not get the Engineer Goby. The neon is a cleaner species, meaning that it will clean other tank inhabitants and some say that this can prevent diseases and parasites such as ich. The engineer goby is a sand sifting species.
Neon Goby: Elacatinus oceanops
Enginer Goby (aka Convict Goby): Pholidichthys leucotaenia