Don't have the 21 - was on my list, until the 20 came along. I fully appreciate the awesomeness of the 21, but the 20 is just right for what I was looking for. I prefer a blade with a straight back, but a forward belly (a la Marine Raider/Western 49 style bowies) for chop/slice.
If for some gawd-awful reason I ever had to stab in desperation (think feral dog, boar, mountain lion), a 'straight' blade makes instinctive sense with a hammer or reverse-hammer grip IMO. Even thought it's huge, the 20 still feels like it could serve as a fighting knife.
That was my original motivation for the Power Eagle over a kukri configuration. However, the Power Eagle was so large, balance so forward, and grip not quite right for those defensive applications (which are mostly fantastical). It's a great knife/sword, and I wouldn't balk if it's all I had.
-However- the BK20 has all the hack/slash feel of the Power Eagle, but still feels like a knife in hand (albeit a big freakin' awesome knife). Also has a more versatile grip, dismountable without a hacksaw, and extended pommel section so you don't mess up your grip material. BK20 is noticeably lighter than the P.E., and I would actually consider wearing it while trekking. So, the BK20 is basically the Power Eagle minus all the stuff I didn't like. It's freakin' perfect.