I was in this exact situation a few months ago. I decided to apply only for the 500 ton and here’s why.
To test for both you will have to take 11 total modules. Rules only once, everything else twice.
Also, for the mates license, you will need the OICNW assessments. My evaluator kept insisting ARPA was a requirement for the assessment also, which my vessel doesn’t have and I don’t have the means of doing the course anytime soon. I was going to then apply for the mate’s license without stcw on it but after finding out about 5 additional modules I stuck with just getting approved for the 500 ton master.