IMO, this is the hierachy for development:
Own 1st team regular, loan 1st team regular (same or 1 division lower), own 1st team regular sub of 20+ mins, loan 1st team regular (even lower division), own U23s/U18s regular, own U23s/U18s regular sub of 20+ mins
There are other scenarios, like a non-playing 1st team squad member but in a mentoring group, but you'd have to treat them on merits. I often have my 3rd best keeper on the 1st team subs bench to allow the 2nd best to develop with gametime in the U23s or even on loan.