Here are a few observations we've gathered from lots of different players over the years. It might be helpful to keep these things in mind as you compare Hammer Dulcimers:
Larger instruments have a larger sound box, and therefore usually have a fuller sound than smaller ones. It's a little like comparing the sound of an upright piano to a grand.
The smaller the range, the more likely you are to outgrow the instrument or find it limiting. If you start out with a 2 1/2 octave, you should probably plan on buying another at some point, because you're likely to want more range eventually. If you want to buy one dulcimer in the hope that you'll be satisfied with it long term, you should probably consider at least a three octave.
Larger dulcimers are no harder to play than smaller ones. In fact, some players find a three octave dulcimer easier to learn with than a 2 ½ octave, because it requires less re-arranging of tunes to make them "fit" the instrument.
A solid top usually sounds better than a laminated top on the same instrument. However, lots of other things affect the sound too, so there may be more difference between two different models than there is between the same model with different tops.
Dulcimers with angled pin blocks, like Song of the Wood, Master Works and some others, can sometimes be more easily and more accurately tuned. We tend to like the more downward pressure of the strings along the side bridge.
Instruments with more than two strings per course can be harder to tune and can have a more dissonant sound. All of ours have two ... or one!
Small, separate (unconnected) bridges under each course of strings can require a lot of tinkering with and moving when tuning. Ours, as well as many other builders, have one piece full length bridges.
Most players rely on position markers on the bridges when playing, but not all dulcimers have them. All of ours do. The visibility of the position markers is probably more important than the visibility of the strings, although there are varying opinions on this.
Chromatic v/s Diatonic:
Most of the chromatic instruments we carry are still arranged diatonically. They just have extra bridges and/or a few existing notes re-tuned, in order to provide the notes that would normally be left out. As a result, they're no harder to play than a diatonic instrument, because you can just ignore the extra notes until you need them.
Most people find that for most purposes they don't really need a chromatic dulcimer. The primary reason for having a chromatic is to enable you to play in keys other than the ones you have scales for (G, C, D, F, and A on a typical three octave.) It also allows you to play in the full range of the instrument in each key.
Even with a typical chromatic dulcimer, it's still easier to play in the keys and scales that are given in the diatonic arrangement, although playing in other keys (or above and below the scales) is possible.
There are also chromatic dulcimers which are chromatically arranged. They have a completely different arrangement from a diatonic dulcimer, so it would probably be very difficult to switch back and fourth between the two. If you absolutely have to play in every imaginable key at the drop of a hat, then this may be the best option for you. Otherwise, most people will probably find the diatonic arrangement (with or without the extra chromatic notes) easier to play.
We don't ever want to encourage our customers to spend more than they can afford on an instrument. We'd rather sell you a penny whistle and have you happy, than to send you out with a $3000 dulcimer that you don't really need. In fact, we often talk people out of buying on the spur of the moment, especially when it's obvious that they're uncertain or feeling pressured to make a quick choice.
On the other hand, we rarely ever hear anyone say, "I wish I'd gotten a less expensive dulcimer than I did," but we fairly often hear, "I should have gone ahead with a nicer one." So, if you're ready to take the plunge, you're fairly serious about it, and you can afford it, you might be wise to go ahead and consider a nicer dulcimer to begin with, rather than skimping now, and then buying another one in a year or two.