Originally Posted by bRiMaTiOn
You use one when there are no antes. You use the other when there are antes.
Not necessarily - if you're playing in a cash game with antes, for example, pretty sure people will still be talking in terms of big blinds.
The important thing is this: as long as one of them works for you it doesn't matter which one you use.
Just be aware that most people use M for MTTs and big blinds for cash games / STTs so you might have to convert when you're discussing them with other people.