Woody: There are some big differences between the Fusions and X-18s.
First, let's talk offset. All three irons you mention (X-14s, X-18s and Fusions) have nearly the exact same amount of offset. The X-18s are sleeker, being two generations newer, and may look like they have less offset. If you want less offset, try the X-18 Pro Series version. They really do have less offset, but are nearly as forgiving. As a previous poster said, try before you buy.
Back to the Fusion/X-18 differences. The X-18s are the latest version of the X-series that started with X-12 and went through X-14 and X-16. Each version gets a little sleeker, a little more playable. They're all cast stainless steel. You won't notice a big difference from X-14 to X-18 in how they play. The X-18s have a notch weighting design that makes them a bit more forgiving than the X-14s.
The Fusions are all-new and different from the X-series. They are a three-piece design, with a heavy body, a light titanium face and a central piece of soft plastic-like material. This increases the perimeter weighting and gives them solid feel. In my experience with them, I thought they were great-feeling clubs. But they feel great on every shot, no matter how bad you hit it. If you're looking for something that gives you a lot of feedback, the Fusions aren't for you.
In sum: The Fusions are more forgiving, more expensive, have the most technology and are the more expensive of the two. The X-18s are solid, versatile and more of a player's club.
Hope this helps.