I spot first. I think read somewhere (many years ago) that because matt board costs almost as much, if not more, than the prime cost of the materials used to make the print, it makes sense to do the spotting first before committing to the additional cost of mounting. But for me, the reason is that a mounted print is physically larger than an unmounted print. When spotting, I want to have the ability to rotate the print at will, and that's easier to do when the unmounted print is smaller.