There aren't two separate models. They're all U21 players - we are selling ours instead of promoting them and buying U21s from elsewhere to promote them instead. Yes, of course we think the ones we're buying in are better than the ones we're moving out, and so they should be - and we hope they turn out to be.Minteh and others who just happen to be U21's were not development signings, hence the cost and were either integrated straight into the first team or sent back out on loan (normally to the selling club)
Players for the U21's who are integrated straight into the U21's are generally a lot younger and cheaper - the two models coexist....
But we're still paying £6M for U21s and loaning them out!