Replace Marathon with Kumho?

I know there has been much discussion on this subject, but I have a question or two. I have three 215-75R14 Goodyear Marathon tires on my 2007 2720SL that I need to replace. In an effort to save a little bit of money and get a D rated tired tire, I am thinking about getting the Kuhmo 857 tire. But I can only find it in a 205-75R14 size--TireRack.com. For those who are using this tire, is this the size you are using? Any comments from those who suggest spending more and going to 15-inch wheels and tires? Thanks.