If you look at the previous five matches, Union Berlin's chances of winning this match are much greater than Leipzig because so far they have only experienced 1 defeat in their previous five matches. The defeat they received was also quite reasonable because they faced Bayern Munich as the top of the standings. Of course, this match will be very important for Union Berlin and even though it is difficult to win, at least a draw is enough for them not to lose full points.
Every match in the next weeks and til the season ends is now important for Union Berlin.
For sure Leipzig hasnt shown there best face the last 2 weeks but it will be hard to get anything out of Leipzig.
For Leipzig its important too , to win that game to catch up to the other teams and maybe have a chance to get close to an CL place.