Too late now, but supposedly the 235s are the single compound construction... That would make them better than the 245s in my mind...
I ended up using the 245-40 R1R's on the back. Single compound construction does not make the 235's "better"....they will simply last longer. Better, in my book, is more rubber on the ground. (At least for autocrossing, which is all they are used for in my case.) I've heard about the 235's, but have yet to see firm evidence of this....just talk from what I know.
To the original poster, I would also comment that while R1R's are indeed a fantastic tire (among the grippiest available in our sizes) they do wear fairly quickly, which was something you mentioned you didn't want. I know you already decided how to go, just mentioning it.