Optional future fees would always be the best option simply because as it states, the purchase is optional. As for mandatory future fees, the contract is agreed before the loan is made official and if they can't settle on the contract at that time, it becomes an optional fee.
If you were to use an optional fee and had to agree the contract at a later date, I don't fully know if his demands would be higher at any point during it but I would assume not.
If I want rid of a player I try to use mandatory fee to get the money in end of the deal, a must if the players contract is running out at end of season.
If I am loaning in a player I prefer to use the optional transfer. The reasons for this are incase they get a serious injury, become unhappy (my loan players tend to be backups) or underperform.
Not sure when the mandatory fees are added/subtracted from transfer funds but believe it happens at the time the deal completes
Optional is always the way to go for me, if you aren't 100% sure you will use this player regularly in your team, or if you might not be able to afford them at the end of the season, or whenever the loan ends. Mandatory is quite risky, but is a fantastic thing to use if you 100% certain you want the player to be part of your plans, as you get the benefit of a loan deal straight away for them to get even more experience and get even better, especially if they are young of course, at a club they are fimiliar with and have good dynamics in.