it would really depend on the individual needs of the team
if you have a training spot open at C, then pump his inside skills first; else do guard skills
i personally would go guard skills first, reason being that higher guard skills will result in a lower salary as compared to higher inside skills
maybe bballin can vouch, but if you want your trainee to make the U21, it would be nice to pump OD to sensational and then round out the rest of his guard skills to around sensational as well
once you are around sensational for guards and depending on whether hes still on the U21, you can either further his guard skills or start moving him to inside skills
where he ends up will be up to you... either be perfectly balanced on all fronts or if you try and bias the build towards more guard or more inside skills