Hi - Is it possible to offer a choice of a discount coupon valid on orders over a certain $$, or free express mail shipping on orders over a different $$, but not both?

I currently have a promotion that gives 20% off orders over $100, and I'd like to offer an alternative choice of free express mail shipping on orders over $50, but I don't want to do *both*.

Should I just do a coupon for free shipping on orders over $50 (will this give people express mail free)?