IMO in the current scenario,using the transfer market is the easier and a faster way to build your team into a winning one.the number of players on the TL is huge and unless you are looking for a player with good secondaries,you can get a very good player off the TL for a low price,especially the big men.
but if we consider the present trend of the decreasing number of managers in BB,in a few seasons the number of players on the market would decrease and that hence their value would increase.at that time training seems to be a better option.
Well this is what I think for a micronation.
But if you are in a big country with a large userbase and start in div IV or V then training is definitely the better option.cuz your player will improve along with your team and if you manage your team nicely,then by the time you reach upper divisions,your trainee(s) will become your starters thus allowing you to spend the promotion bonus and other income on strengthening other positions.