The 260C (maybe all 260's?) have a longer fuselage (and an additional window) that accommodates the third row. I don't think the 250's or 400's have the room behind the rear row.
The fuselage was the same length on all the Comanches.
I have flown both. If you need six seats, you need the Six. The 5th and 6th seats in the Comanche are only suitable for small children and take up ALL the baggage space.
The Six can be compared to a Chevy Suburban while the Comanche is a big Beemer, in the way they both fly. If the Six is older than a 1978, the speed difference will be 20-25 kts in favor of the Comanche. You will seriously love hand flying the Comanche. I haven't met anyone with experience in a lot of different aircraft that rate the Six as sexy to fly.
The Comanche is better built. They are stronger and have better corrosion protection. The negatives are that they are more expensive to maintain and it is harder to find a mechanic who knows what they are doing with a Comanche. You have to be more involved as an owner. Connecting with the Comanche fanatics is a near necessity as the tribal knowledge is vast and usually exceeds what the professionals can offer. Insurance will be higher in the Comanche.
The Comanche is a much better high altitude aircraft. It is really happy at 12-13K where the Six starts to wheeze above 10K.
The Six is better on shorter and rougher fields than the Comanche and it has a higher useful load. The cargo door is hard to beat. That gives a lot of flexibility for hauling that the Comanche does not have.
As for these specific planes, I prefer the high time engine and avionics over the low time engine. The low time engine is a gamble. Depending on the quality of the overhaul and how much it has been flown, you may or may not make TBO. Odds are probably 50/50. The high time engine doesn't owe you anything. If you get a couple of hundred hours, great. If not, no big lose. When the engine is overhauled, you know what you are getting. Also, the cheapest way to buy avionics is already installed.
If you need help rationalizing the Comanche :wink2: you can find the fanatics by Googling "Airworthy Comanches". It is a forum over on Delphi. I know, as I am one.