I run an aftermarket 18" wheel to replace the OEM 19". I still have the 19's, I just never use them.
I'm kinda old skool when it comes to wheel size, especially on a 4WD vehicle, so I don't like the larger rims much. Waffer thin rubber and huge wheels are for race cars and rap stars IMO.
My LR3 gets driven on a lot of dirt roads so the 18" rim gives you more rubber between you and the road. A 18" wheel is as small as you can get away with on a Gas V8 LR3 because the brakes are so big. In markets where the diesel V6 is offered they can fit a 17" wheel on that model due to a smaller brake/caliper size.
It is worth noting however that all the European Land Rover Experience centres and the trio of LR4's that traveled from Birmingham to Beijing all had 19" wheels with the truly excellent GoodYear Dura-trac's. I love this tyre and run it on my Rangie and 3/4 ton Dodge. Not sure if they are available here yet in 19" sizes or not, they were being produced in Germany for the European market?
Since swapping to an 18" wheel size the only differences I have noticed is a more comfortable ride, greater tyre fitment options and way less punctures.