At least to me, formulas aside, training facilities are a better choice....
They affect all your squad: it doesn't matter if you have 5 or 50 players to train, 1 coach can train at full strenght only 5 players, if you give him more, he will not do his work right.
Maintenance fees are far less expensive: Only one 700k coach takes about 15k weekly, so if you hire 2, you must consider your expenses will increase about 30k each week, training ground in good level takes 12k each week.
They can be upgraded without paying any compensation (if you have 5 coaches, you have to fire one, and must pay for that).
Honestly, having the money and formulas aside, i think training facilities are the better choice.