Pure centres (without guard skills) are a dime a dozen on the market. So if your training a big man your better off training a PF. Training a SF could give u a big advantage because they are expensive to buy and can give your opponent matchup problems. However training a SF can be difficult because a lot of the time u have to play them at PG or C for training purposes.
There is usually a good range of SG available on the market. There are fewer PG's available and Passing seems to add a lot of value to a player. So if your looking at market availability training a PG or PF is a good option.