It depends on what you want to do with the doped wafer. If you will do the surface micromachined MEMS (i.e. deposition and patterning, no substrate etch), then you don't care about the doping type. If you are going to do the bulk micromachining (like controlled TMAH etch from the back side of the wafer to form the membranes on the front side), you will probably want phosphorus doped wafer (n-type), since boron doping (p-type) inhibit TMAH etch and sometimes is used as an etch stop in bulk micromachining.