check out
http://www.1010tires.com/tiresizecalculator.asp
245/35R18 is closer to stock diameter so your speedo will be a bit more accurate. Also, the difference in overall diameter between 225/40R18 and 245/35R18 is only about a quarter of an inch, so an eightth of an inch on each side of center. I don't see that making a huge difference in ride between the two.