I made the switch from Nikon to Fuji a few years back, never felt I was missing anything, Fuji cameras and lenses are top quality. The 50-230 isn't bad, it just needs to be used in good light, it's not the best build and maybe not the sharpest lens, but if you only want the reach for casual use it's perfectly fine. The 55-200 is a superior lens, it's much better built, has better OIS, it's a fair bit brighter at the long end and it's a bit sharper. It depends on the price you can get it for, it's a little high new, but can be got for about £400 used.