Of course, the most successful sales forces will be well chosen and well trained. However, if you have to pick which is more important, it is selection.
The reason for this is that sales is a profession that demands a certain personality type. A person who is shy or who has a hard time making people like them is not likely to ever be as good of a sales person as one who has more natural charisma. This is something that cannot completely be remedied by training. For this reason, it is much more important to choose people who will make a good impression on customers. Poorly trained people will winning personalities are more likely to do well than well trained people who simply have the wrong personality for selling.