I have also owned both, and currently use two JC1s with B&W 803(d)s. I agree with Thankful in that the JC1s sound considerably better. The JC1s have better bass articulation and a sweeter high end than the A21. More detailed and better transient response. The high bias setting sounds particulary good, and for most program material at reasonable listening levels the amps are running Class A. The only downside I have found is that in high bias, the JC1s are sucking 450 Watts continuous from the wall.
As far as power goes, I had no problem driving a previous pair of 804(s)s with the A21. I would think your 802Ns are at least as efficient as the 804s were.
I think I calculated at one time that two A21s in bridged configuration could deliver more power cheaper than two JC1s, but they won't sound as good. Plus, there may be stability problems running A21s bridged into an 802 because these get down below 4 ohms at their minimum impedance, and I don't think you can run that low of an impedance into bridged A21s (min 8 ohms bridged). The JC1s can run as low as 2 ohms, so they are great for the B&Ws impedance wise.
As far as cost goes, wait for some used JC1s to come up for sale here on AGon. I bought mine used here in pristine condition for $3600 for the pair. At that price they are a bargain.
The only issue I have had with the JC1 reliability wise is the surge current limiters on early production units. In one of my amps, one of these nearly caught fire because it was undersized in the original design. I think Parasound will replace these for free with upgraded units (they did for me).