From one beginner to another, a couple of things. First, if you're in the Seattle area, snap up this piano dulcimer on Craigslist: http://seattle.craigslist.org/skc/msg/2081784318.html
A PD-40 with case, stand, hammers, and dampers for only $500! Almost makes me wish I a) weren't learning the regular hammered dulcimer, and b) lived on the other side of the country!
(This probably proves your point about the resale value, as I saw it not sell when listed at $600.)
Anyways, on to business. Some people might remember me as the person asking a lot of questions on Everything Dulcimer about piano dulcimers (I eventually got a Dusty Strings D35, however, which is three octaves in the traditional fifth-interval tuning). Like your wife, I had piano experience, a good background in music reading and music theory, and no hammered dulcimer experience. I will say that once you actually try a fifth-tuned dulcimer, the layout is a snap--for me, it was even easier playing the actual instrument than when I was trying to map out hammering patterns by looking at a tuning chart. Even though I had experience with a linear note layout (piano) and no experience on a more pattern-based instrument (e.g. guitar), I'll bet the normal layout was easier to learn than a piano dulcimer would be.
However, one of the pros of a piano dulcimer: it is the same price and the same size as Dusty Strings' D45 model, but it has way more notes (an extra half octave of range and it's fully chromatic throughout unlike the D45) and slightly fewer strings to tune. It is also as easy to play in Gb (which only a few people regularly do on a standard hammered dulcimer) as it is in, say G major. I personally was intrigued by its ability to play in all 12 keys.
The downside, besides the resale value, is a couple of things. First of all, I know most hammered dulcimer players recommend you play "horizontally" rather than "vertically" wherever possible. Basically, it is easier to hit the right note if you move over to the right and hit it on the bass bridge than if you move down five notes on the right side of the treble bridge, for example. You can't do that very much on a piano dulcimer. Second, you don't get the diatonic patterns that lend themselves to a lot of cool ornamentation and are easy to improvise in (and that most instructional material is written for, for what it's worth).
For me, when I was choosing, I basically thought about why I wanted a dulcimer. Did I really need something that was going to have the range and chromaticism of a "normal" instrument? Let's face it, a guitar has more range than your typical hammered dulcimer, and is still more portable, and has about a tenth of the number of strings to keep in tune. And a piano has several octaves of range even on a 5-octave JRS Concertmaster, and is fully chromatic too. There's a reason why the orchestral instruments, the typical concert instruments, are orchestral instruments, and the folk instruments are folk instruments: they are more versatile. So since I was really intrigued by the sound and playing motions (I love the feel of the hammering, and it's also fun to watch) of the hammered dulcimer, I figured I would go with the more easily available and the more traditional instrument and just make the most out of what I could.
Plus, there are people who play pretty intense music even on a traditional hammered dulcimer, so it's not like a non-piano dulcimer has no musical capability at all.
That was probably more than you needed to know and off-topic anyway, but in case it helps...
Good luck! Hope your wife enjoys whatever dulcimer you get for her!