I read your question carefully and I understand your concern.
I don't think you should worry too much though. It happens after a
concussion to have some residual headache which usually resolves inside a 3 months. Since you have had a
CT scan there is no reason to panic. The
sinusitis might have been present before, it's a coincidental finding if the headache came after the
injury, it certainly wasn't caused by the hit on the head, so shouldn't rush into unnecessary treatments and tests.
As for which is better between
MRI and CT it depends on what one is looking for. Generally MRI is more accurate, but if one wants to evaluate the bone then CT is better, it's also as good to identify acute bleeding as MRI. MRI would have more value if a condition involving the brain and soft tissues was suspected, judging by only the info that you give I don't see a reason to suspect that.
I hope to have been of help.