I am looking to replace tires on my 18inch to more streetable tires. I wanted to buy Hankook RS2s but from what I read they have soft side walls. Should I just go for stiffer side walls to help protect my wheels? Or would protection from stiffer sidewalls be very minimal? I would like to keep the diameter as close to stock as possible, so I will be running:

225/40/18 fronts
245/35/18 or 255/35/18 in rear.

Perhaps getting 245/40/18 would be better protection?

 

Trying to keep the overall diameter the same as stock will always limit the choices you have for aspect ratio given any width you choose. On an 18" wheel, you have to run a fairly low profile (low aspect ratio) tire. Other than "run flats", you're not going to see a huge variation in sidewall stiffness. Upping tire pressures may be your only other alternative, at the expense of increased center tread wear.
Of course, you can go with a taller tire (assuming you don't have the car lowered too much) manage the speedo error in other ways. You'll also sacrifice some torque.