kill gentleman jim stacy?

whats the big reward for finishing the FG questline?

The final quest is either to kill Gentlemen Jim Stacy, or to kill Sjoring Hard-Heart... depending on how you do the questline.

The reward is to be named Master of the Guild, which has the usual disposition boost among the Guild members.

But... working for Percius grants a nice bonus of being able to enter his room and get some Orc and Glass armor. (as long as Hard-Heart is alive)
While working for Hard-Heart means you have to battle him for the title of Master.

...and you don't have to kill Percius. He's glad to give you the job.