First off: I don't know if I could reliably tell one from the other, if blinded.
Sighted comparisons...Beware of Loosey Audiophile Terms:
Ember has plenty of power for orthos and lots of spare headroom. It edges out the a-100 on clarity or definition, for lack of a better term. Very smooth with no grain. I like it better than a-100 with both types of headphones. I think Ember digs deeper with better bass extension, quality, texture, and speed. Ember sounds better than a-100 with HD-800 which sounded a bit hot out of the a-100. I found myself wincing and reaching for the volume pot due to discomfort in the upper register on some cuts. I have a NOS Telefunken E88CC tube in Ember - my favorite tube. It's very pricey but there are other tubes available for little money ($15 to $20) that sound almost as good. The Russian tubes come to mind.
Emotiva a-100, modified, also has power to spare. It's performance was generally satisfying with both types of headphones. It offered a bit less resolution than Ember but this was with sighted comparisons. I think I could tell them apart in the "clarity or lack of grain" department, though.
Emotiva a-100 is plug and play...set it and forget it.
Ember offers tone shaping options by rolling tubes. While tube rolling is fun, it's time consuming and costs more initially and in the long run than the solid state Emotiva.
With all this in mind, my personal preference is Ember for its sound quality, tuning capability, power, clarity, small form factor, and its cool color glow. Yeah, I'm a sucker for the changeable LED colors. The deep purple glow in a darkened room somehow adds an extra layer of pleasure to my listening experience. But, hey, this is psychoacoustics...if it sounds better, it Does!
If cost is a factor, and I assume it is at this mid-fi level, I would buy the a-100, modify it with robrob's design, stick to planars (my preference), and "Be happy...don't worry ;-)
If I wanted better clarity, tone shaping options, and multi-sensory experience I'd pay just a little bit more and get the Ember..... and never look back.