Hey Bud
This link takes you to the Geo for the 303.
http://www.yeticycles.com#/ourrides/303_DH/TEAM/The frame size will be pretty much the same as your Sunday but the top tube will be 1 inch longer. Im 5ft10, and Ive got a medium sunday, and Ive ridden my mates medium 303, and the sizing feels almost identical. You would be best to try the 303 first I think, due to the longer top tube.
If its a pre 08 frame, then Im pretty sure they were only 2 sizes, the only difference being the top tube size. If you and unsure give Bromley Bike Co a call as they are the importers for the UK, and super clued up.