Please talk me out of a 31 mm ltd... or?

I have the 15, 35,43and 77 limiteds. I went to B&H one day, hell bent on getting the 31. They only had one copy left of lt and this particular one had bad pp (with my K5) plus it seemed decentered, I could not get a sharp shot at the store. The sales manager offered a very nice price, but comparing it with the 35 2.8, it didn't seem as such a great value. I love my 35, plus it's a macro to boot. The only reason to go with the 31 is to be ready for the elusive Pentax FF some day