Lemon Coriander Weiss hops substitution

[Sharpie]

I'm about to do the Midwest Lemon Coriander Weiss extract kit and I have an odd situation. The recipe calls for 3/4 oz Saaz for bittering (60 min boil) and an additional 1 oz Saaz aroma. Instead of sending me 2oz Saaz they sent me 1oz Saaz and 1oz Challenger hops. I've never worked with challenger hops. Should I use those to replace the Saaz for bittering or aroma?