I've used Sonax Gummi Pfleger on both cars and it works well. I think the 303 is more of an anti-UV coating, that would stop discolouration of plastics due to sunlight.
I half opened the roof and did all of the seals, front, side and rear, but don't apply the Gummi to your window seals, it'll leave a greasy mark. Also, once applied, I think it's worth giving it a squidge in with your finger and thumb, to make sure it's applied evenly, and let it dry for a few minutes before you close the door/boot/roof.