I now have the choice to kill Trevor or Michael or do Deathwish. But the thing I wanna know is, do I still get the money from 'The Big Score' heist if I choose Deathwish?
I'd recommend you saving the game before the mission and replay it with the different endings. That being said yes you'll get at least 20 million dollars either way:
In any case you get a lot of money. If you choose to kill either Michel or Trevor, you'll spend the end mission killing them, after that Lester will give you a call (I only killed Trevor but am assuming its the same for Michel too, but I guess his family gets the money not Frank) telling you he can't reach one or the other (depending on who you killed) and he'll transfer you (Franklin) the killed character's cut too, so you'll get 40 million dollars. If you choose Death-wish, everyone lives, everyone gets 20 Million and you kill some other dudes instead.
One thing to note though:
If you choose to kill one or the other, they are obviously dead after the mission, and are unplayable, you simply can't switch to them.
So yeah, that's that.