To be specific, every training allows you to transfer 0.25 of player's every skill, if higher than 0.25, into main skill; 100 of second position, if higher than 100, into main position.
With that being said, it's much efficient to convert a midfielder (normally with equally skills on passing, dueling and tactics and higher second position) than a goalkeeper (usually with only blocking and one position). I would admit a player with over 1,000 position experience is better, but once he's beyond 1,000, his position experience would gradually decrease with every match being played...