I am not an audiophile so I dont know if there is a measurable difference between the sensitivity rating, or at least a difference that one could hear.
As to marine grade--I haven't found myself in a position yet where I am glad I have marine certified speakers. Even being caught in rainstorms I never noticed the dash getting soaked. I suppose it does provide a nice warm feeling though knowing they are marine certified.
FWIW--Kickers are in stock, Polks are on backorder. At least according to Crutchfield.
And another FWIW--I bought everything (head unit, the 522s, the 652s, sub, amp and headunit) all through Crutchfields scratch and dent site. The only thing I didnt buy thru them were the speaker pods (found those on ebay, also scratch and dent). I saved quite a bit of money going that route. Its a jeep that sees dirt and gets beat on, I didnt need anything fancy and scratches/dents didnt bother me.