Dack's +1 is probably stronger than Jace's +0, because it digs deeper and is a plus loyalty ability, rather than loyalty neutral.
Eh.. What?
Yeah, I know. Sounds crazy. You have to try it to believe it. You could trust me on it; but really you should just try it out yourself.
Edit: you can certainly play both Jace and Dack. Which is actually great cause you can just use Dack's +1 to fix your hand and it leaves Jace free to protect your walkers with the -1 or to use the +2 as a win-con.