Oh, wow. While I love the look of the Clydesdale, replacing the Cat every 10,000-12,000 hours or 3 years would be a big downside in my opinion. Looking online it looks like the OEM Cat from Hearthstone is $357 before tax/shipping. Also, that's $357 per cat and my understanding is that there is 2.
Say if I had the insert for 10 years that's over $2100 in just the Cat alone, not including other parts that might need replacing.
I do like the look of the Lopi Medium Flush insert a lot more than the Evergreen but unfortunately that model doesn't seem to be covered in the tax credit.
The catalyst is Hearthstone part #3050-010, which can be found in hearthstone branded replacement kits of 2 for ~$400 online.
It's also an Applied Ceramics ACI-6M2. It's a standard form factor cat used in a number of other products. The 6M and 6M3 may be drop-in replacements as well.
The 3rd party Midwest Hearth MH-6M appears to be the same product, for $120 each.
By the time these things start needing to be replaced more regularly I would expect to see them become more widely available at more competitive pricing.
The life of the cats in these stoves is still unknown, however, I would expect them to be longer than 3 years in most applications.
These are hybrid stoves, not pure-catalytic stoves. Every hour of stove operation doesn't equate to an hour of catalytic combustion when operated at recommended burn rates. A typical burn cycle in a Hybrid design only transitions the wood-gas combustion to the cat during the transition from active flames in the box to coaling. There's a period there where there's still hunks of mixed charcoal/wood lumps still in the box, but they are not off-gassing fast enough to sustain active flames anymore. That's when you will hear the distinctive "tink tink tink" of a catalyst ramping up and down.
In my experience, the catalyst in these stoves is taking over as the primary point of wood gas combustion for around 1-3 hours out of a typical 6-12 hour burn cycle (I burn softwoods here), and is otherwise playing "cleanup" of a very small percentage of combustion gases during the first 2-4 hours of that burn cycle while flames are active in the firebox. If I burn 2 cycles a day, (1 late afternoon/evening, then an overnighter), that's about 4-6 hours of actual "combustion-time" per day. Assuming an average of 2 burn cycles per day for 6 months a year, that's around 1000 hours of "combustion" time per year on the cats.
Time will tell, but I expect these will last more like a decade in this application, and I would expect the cost to replace them in a decade from now to be ~$300 equivalent (inflation will probably make is $600, but everything else will also be double).
With all of that said, the Lopi in this case appears to be a better stove design. It achieves efficiency comparable to cat stoves, without a cat, reasonable emissions, and a wider range of thermal output. Personally if I were buying an insert I'd be inclined to avoid cats as they will be more difficult to clean/service in there. A secondary tube combustion system is going to be the more trouble-free approach.