If i'm projecting their roles, I see Smith as a 2C and Leonard as a 1RW.
I don't think Smith will be well-rounded enough to be considered as a first line center, although I think the offensive flair is there. He'll put up points (100 is definitely possible), but I don't see him being a dominant force. For instance, I think the stickhandling/shot/vision is on par with Jack Hughes, but the skating is so far that I don't see Smith reaching that level. Looking at the league, an average-skating center needs to be either big or extremely responsible to be a 1C.
Leonard will make life easier for skilled players on the top line for sure, but people tend to underestimate his own skill. He will create offense.