This shoe seems lighter and more flexible than the 18.  I have the problem of one foot larger than the other, so while this fits my left foot perfectly, I needed to work at it a bit to get the right shoe to work - but the shoelaces are aligned just right so that tightening one and loosening the other prevents slippage, making them both very comfortable.
I run 15- 20 miles a week and have bad ankles and knees, but with these shoes (and the 18s) I am able to run without too much pain and no swelling afterwards.  The only caveat is that, if you try them on in a store, the insoles that come with the shoe make the arch seem painfully high, especially if you have flat feet (and these are the shoes they recommend for flat feet!)  A decent sports store, like Sports Authority, will let you pull out the insoles and put in some of the gel insoles so you can eliminate that stiff lining they come with.
Then, of course, come back and buy them on Amazon.  :)