I went 9-10 k happy miles on the PR2. (some wear left on the front) Replaced them w/ PR3 and now have 5500 ecstatic miles on them. The 3 is the same rubber compound (said the Michelin guy @ IMS) but w/ added sipes galore!!!!
My estimate is similar (road) miles on the 3 vs the 2. The difference is the sipes.....they add stick to the entire tire, reduce heat, but also reduce rubber on the road, so faster (potential ) wear on the sides.
Since they have the same profile, the (theoretical ) same compounds, and therefor the same mileage.....I would always get 3s vs 2s because you may/might/will get caught in the rain. They clearly are better in the cold, wet, slippery surfaces, so why not buy that added safety and confidence. YMMV