Dermatologist or Rheumatologist?

I do not want to appear flippant, but the best way id for you to ask them. Someone will need to monitor your body for any adverse affects of the methotrexate, via periodic blood tests.

"which of these doctors will take the decision on dose and how are decisions made regarding changes in treatment?" It is probably going to depend on which condition is affecting your life most at the moment. I have both, but it is my rheumatologist that is driving the decisions on my treatment because it is the arthritis (PsA) that has been having the biggest impact on how I am able to live. If you had very mild arthritis, but skin psoriasis was having a material impact on your life then the dermatologist would be driving decisions. Having said that, while most treatments for PsA have a beneficial effect on skin psoriasis, the same is not true the other way round (particularly of topical treatments). Your dermatologist might therefore be quicker to recommend DMARD treatments if you also have PsA.