As far as cost effectivness, buying equipment is a better deal than training or buying players, as it gives your entire team .5 stars. All of the players stats are based a a scale of 0-5, in half step increments. They also are assigned a useless overall rating. When training players, about one third of you players get one full star, but players that already have 5 stars are included. This means that you waste a lot of money training for 5s. Every half star costs a specific amount, speed costs more than the rest, weither you buy players or equipment. Buy your team equipment.
The reason that some players seem faster than others is realated to positions, #s 9 and 10 are fast, while goalies are slow.