What those three dulcimers have in common is that they are all high quality instruments that retain their value well. But they are all very different. I've not played a Warren May dulcimer, but his instruments probably have a more traditional sound than the other two. McSpaddens are very well balanced and are famously easy to play. Blue Lions have a bigger bass sound than McSpaddens. But there are other small differences, too. The McSpadden fretboard is a bit smaller than the Blue Lion fretboard, and Blue Lion strings are a bit further apart. The VSLs on standard models are also different.
If you have lots of money, buy a dulcimer by each of those builders, play them for a while, and then send me the ones you don't love.
If that's not an option, wait. There are so many variables in dulcimer builds that unless you've been playing for a while and know exactly what you want, you don't know what your preferences are. It took me a few years of playing to determine exactly what kind of dulcimer I wanted (and I want different ones for fingerpicking and flatpicking!). Take your time. If you can visit Warren May, play some of his dulcimers. If you are at a festival and see a McSpadden or Blue Lion vendor, play some of their dulcimers. Whenever you hear someone play a dulcimer that sounds especially nice to your ears, ask if you can play it, and ask what the player likes about it. Eventually you won't have to post the question with which you started this forum because you'll already know what kind of dulcimer or dulcimers you want.
Dusty T., Northern California
As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie