Mill Street and Opal are both far from Headingley Campus, either way, you're getting a bus. Go for Mill Street if you're okay with sharing rooms. Theres usually a 2 girl to 3 boy ratio in each room, girls bathroom, lads one, simple. Mill Street is a lot cheaper than Opal and people who stayed there have been here and said it's just, if not a lot nicer than opal.
Thanks
Scott