Big difference between the two bindings. I believe the PX-18's have 6-8 mm of delta between the toe and the heel piece. The Dukes will probably work out to zero delta for an alpine boot. Most alpine bindings are probably going to be in the 4mm range. I would not recommend buying a Duke without actually skiing one first. You may find that the zero delta dramatically changes the way the ski behaves and you may not like it. With my old boot setup, the Dukes put me in the back seat and were borderline unskiable in anything other than deep snow. I haven't tried them yet with my new setup, but I'm hoping for better. BTW (Lou Dawson's site claims that Dukes have a 8mm ramp angle, but he was probably measuring using an AT boot). Mine measure to zero and I have yet to see anyone with positive ramp on a Duke using an alpine boot. Here is a thread on TGR discussing the Duke delta issue: http://www.tetongravity.com/forums/showthread.php?t=120776
If you don't plan to do AT, there is no reason whatsoever to buy them. You are just exposing yourself to needless headaches and more parts to break (as well as spending considerably more money for the privilege). That said, other than the delta issues, the binding is very solid (if heavy) and I've had no problems with pre-release on mine.
I have PX-12 bindings myself and I like them. The Look design is rock solid and I've had no problems with pre-releases. Some folks don't like the Looks because the delta is agressive, so again you might want to ski them.