c18 is the stronger of the two, nevermind, in fact, their about even.
Anyways in android saga, Piccolo was the strongest fighter up untill cell became imperfect cell. I mean, Vegeta would've got his ass handed to him by c20, while picollo gave him a fight for his life. Picollo also was the one who handed cell's ass to him, and c17. so super namek > super saiyajin.