Well the KDX is a pretty tough bike. there is a bit more top end maintenance than a 4 stroke required overall, but your maintenance schedule with a KDX will be comparable to a KLX.
Your brakes, bearings, chain, sprockets will all be on the same intervals. just the top end is different. You have to keep in mind that the KDXs have been around for a while with very few changes. So parts are plentiful and cheap. Once you learn to do a top end, it is like 1hr work and $100 at the most (if nothing is damaged).
The KDX is lighter than the KLX bikes and is cheaper to modify for more power. With a KLX, the first thing you need to do is get a good pumper carb, $400. Well with the $400 you can have new aftermarket reeds, pipe and silencer with some pennies left over on the KDX.
For fouling plugs, any bike can do that if it is not jetted properly or has bad rings. You may need to change the jetting a bit more with the KDX 2 stroke with varrying conditions, but it is a simple task. Always carry 2-3 spare plugs in your pack incase you foul a plug or two out riding. ALso using the proper mixture of gas to oil is critical to keep from seizing the bike or fouling plugs. Invest in good 2 stroke oils here, I run the amsoil personally.
Both bikes have good traits in the woods. That is where the bikes are at home. It pretty much comes down to a personal 2stroke vs 4stroke issue here.