Those aren't bad prices at all. Was that from their site/product catalog? I wasn't able to find the higher end stuff there, just street challenge. I may be searching incorrectly or something...
17" wheels are lighter and cheaper so they have a small advantage there. The real advantage for me is on the tire end. There are a lot more tires available in the 245/40/17 and 255/40/17 size than there are in 245/35/18 and 255/35/18. The prices are also a lot lower on the 17" tire setups. Marginal benefits also include the increased sidewall height for a much more comfortable street going ride as well as extra wheel protection when dropping off a rumble strip or edge of a track. I run Direzza Z3 tires currently (took a chance on them early), and they're not exactly "comfortable" in a 245/40 when inflated to spec. I can imagine what a 245/35 would be like with a stiff tire...probably similar to my old Hoosiers. 12mm sidewall difference can be pretty huge.
I run 235/40/18 on my nuespeeds. And yes their catalog isn't online so I got that from talking/emailing them directly. Similar price points to other makes but they are on a summer sale till 7/31 so it makes it that much more attractive to me vs stoptech. (Add shipping $100-$200 onto those numbers).