Personally, I liked the chemistry between Riley and Sulu and I always found Chekov to be a bit of a bore with his "inwented in Wussia" nonsense. I realize that Koenig was brought in because of the whole Davy Jones thing, but I often wonder if the show wouldn't have been better served had Riley been kept on as the recurring navigator. I find that Hyde brought a great deal of youthful energy to the episodes in which he appeared and I think his character could have been developed further.
What do you think?
Taking the question in a vacuum*...
The Riley character benefited strictly from being a 'guest character' for two episodes, which provided extensive screen time and script focus, both of which provide ample opportunity for character depth to be shown. Chekov by comparison had a few moments of genuine focus but all in all he was about as important plot-wise to Sulu which in turn kept him on the sidelines and have him few moments to have a genuine emotional impact.
Put Riley in that particular role, and maybe he has a better chance to better develop his character just based on what he did beforehand in those two episodes. But honestly, I think it's a wash.
*- Based on the meager reading material I have in regards to the making of TOS, Bruce Hyde was contracted for a Pilot at the time but could be used for up to two episodes on any existing TV Series that Desilu had. Star Trek turned out to be those two episodes, if I'm not mistaken. (I'm sure somebody browsing IMDB is gonna correct me)
That said, Hyde was also primarily a stage actor and while he acted up until 1970, I'm reasonably sure he stuck to mainly stage as opposed to television. So his availability or willingness might've been in question.
Another factoid to remember, and this is just to bring anyone else up to speed, Chekov wasn't meant to get the amount of screen time, or screen relevance, that he did get- he was essentially getting what was meant to be had by Sulu- George Takei missed a sizeable amount of time during the Second Season because he had a role in Green Berets and they went overschedule.