slightly concerned about diagnosis

Hi
I've recently been diagnosed with Rheumatoid or Psoriatic Arthristis. However the symptoms are not typical. I was originally diagnosed with PMR in January and put on prednisolone (by my GP) which worked well until I got down to about 2mg. I was referred to the rheumatologist and saw her once I was off the prednisolone for a few weeks and she feels RA or PA is more likely. I think this is primarily due to my age - 51 - as pmr is rare for one so young?! and partly as the symptoms were also not fully typical of pmr. Thus I'm about to start methatrexate.

My symtoms are:
-pain in my shoulders, worse on the right, and particularly when I extend my arms backwards of upwards - in this case the pain kicks in a second or so following the movement.
- pain in my left wrist (though not much at present)
- stiffness in my neck, partcularly if I turn my head to either side
some stiffness in my lower back and the back of my legs
- sciatica after about 45 minutes in the car

I have no swelling or pain in my hands or feet or anywhere else. I had a mri scan on my right shoulder which showed some impingement. The orthopaedic surgeon said he thought it likely I'd have impingement in both shoulders.

I'm a bit concerned I'm being put on a strong drug and that, if I do go on this, my diagnosis should be correct as far as it can be for these conditions. I'm not really sure what I'm asking here, but does anyone feel I should push harder for further investigation or simple go on the methatrexate in case it is RA.