I actually fired up the car when I was only done the one side and I really could not tell much of a difference when I fired it up after doing the second side. I'm sure it's probably a little bit louder with both sides done.
I was going to just do one side but then I felt weird, like it was wrong to not do both, so I just went ahead and did it.
After driving around a bit today, I'm definitely happy with it. (I used 3 washers on each bolt, both sides)