One thing you can do is ask the students to decide what type of assessment they want to take beforehand (i.e. submit a decision in writing by a certain deadline), create separate assignments for each type of assessment, and then use the "Assign To" feature in the settings. There might be an easier way but I'm not sure.
Thanks for the suggestion but this subject will have over 1600 students next semester so assigning each person manually wouldn't work. I think it's the same problem with Sections.
I was hoping to use Groups, as students can self allocate, but "Assign To" doesn't include Groups as an option.
If those are the only assignments in the course, you could use assignment groups to get the weighting. You'd have to have two copies of the exam and expect students to choose the correct one for their grade.
It's risky, but it may be the only option. I've been searching for an in-system solution for a couple of days without any luck.