In my opinion, Get the box blade first. Matter of fact, If I could only have one 3-pt attachment it would be a box blade. There was an interesting post on this forum a while back how one poster used a box blade for several alternative things from plowing snow with it to using the rippers to till a garden plot and the list went on from there.... The list of tasks that he was able to do with that one attachment was amazing. Granted it may not be the absolute ideal tool for several jobs, but it will still work for many things. A regular blade is much more limited it what it can do.
FWIW, I actually like plowing snow with my box blade better than my regular blade. Now if I had a really long lane the story would be different, but the box blade works better for my situation in snow. Since I never cut ditches, I am actually thinking about selling my regular blade as I never use it.