I went through a wheel/tire upgrade, then later a suspension upgrade. I went from a 235-55-16 to a 265-35-18 front and 275-35-18 rear setup. Just changing the wheels and tires gave a good upgrade to the responsiveness of the steering and overall grip in corners. The ride was slightly stiffer, but it really wasn't a major change.
However, when I upgraded the suspension, especially the sway bars, it started to ride like a go-kart. I don't recommend getting sway bars to anyone unless you like to get beat to death at all times while driving. I'm thinking of going back to the stock sway bars soon. Springs didn't have that much effect on ride quality though, just a slightly shorter travel to bottoming out.
In summary, just get the 18's you know that's what you really want. Don't have regrets.