If you are talking about 18y olds, I think it's like this:
1 pos.: average 13 pops x 3 players = 39 pops
2 pos.: average 10 pops x 5 players = 50 pops
So you would get more pops with 5 players, but none of those 5 players would be as good as any 1-position trained player.
Now 3 pops isn't much, but over 4 seasons that's 12 pops...