An update for me, I no longer own a Boss or Beagle but having owned both I'll throw two cents in.
The Beagle is throwier than the BOSS, where the BOSS sort of makes of for that, is in much higher drive current which allows it to brute force for a short range, but at the expense of very short battery life and high heat.
Ergonomics, the BOSS feels better in hand, particularly with the 70 body which is actually pretty damn compact for a rear switched 18650 setup, the problem here being that the Beagle is essentially a straight tube, there isn't much aside from the clip for retention in the hand, and while the knurling is nice, it doesn't help as much in grip as I would have liked.
Clip, the BOSS uses OR's SS clip which I absolutely love, and will say it is the single best clip available today, great tension, smooth, slick look on most anything, and cheaper than most any of the other standard offerings. The Beagle clip is well...atrocious at best IMO, stock they are too weak for such a chunky light, sharp-*** edges, and poorly finished, especially the optional Timascus offering that were just roughly milled, bead blasted, and heated for a finish and color selection that looks more like mud than usually colorful Timascus. With an appropriately sized bushing, you can use an OR universal clip for much better retention, but doing anything to your Muyshondt light besides changing the battery, voids the warranty...
Battery selection, they can both strictly speaking handle 18350s, 16340s, and CR123s, though the BOSS requires some programming to achieve that.
UI, BOSS is programmable but is essentially up to 4 modes with an auxiliary color LED option, the Beagle is not programmable but is KISS with a simple 5 mode selection from low to high with the first couple modes being strictly the flood ring.
Warranty, hands down the BOSS is the better light to own in this respect.
Now, don't get me wrong, I love the concept of the Beagle, I just think as it is implemented it is mediocre. A proper dual beam light in the custom area is something that just doesn't exist aside from the long discontinued Lunasol, the problems with the Beagle IMO, are bland/slick host design, poor clip, too small of a center reflector, use of an XP-L2 in the center instead of something throwier and/or with better tint, and a loosely fitted engine, the pill is fitted loosely into the head with some thermal paste, no threading on the production lights. I hope to see a better revision or version of the Beagle concept as it has promise.