Medium telephotos are generally my preferred lenses for product shots. I think the 180 or 250 would work quite well for your violins. Then again, you already have a 90mm, which does the job just fine. Maybe I would concentrate more on the setup and lighting quality for now than on getting a new lens. IMO both of these things could be better in the pix. A longer lens would eliminate more of the background, which would be nicer, IMO.