You might want to get a hold of the Aviation Consumer Used Aircraft Guide:
http://www.aeroprice.com/store/uag9th.htm
Come to the Nashville meeting and talk with some Skymaster owners in person:
http://www.337skymaster.com/nashville2003.htm
You might also consider attending one of CPA’s aircraft systems courses. There’s a 210 course coming up in May – no more 337 courses for this year. They can tell you a lot about the model history and quirks of the aircraft you are considering:
http://www.cessna.org/benefits/cours..._schedule.html
Some of my observations about these aircraft:
Pre '76 Skymasters have a "fixed" rear seat. It does not slide forward or backward. However, it does pivot up out of the way to allow much easier access to the 5th seat and/or baggage area.
And speaking of baggage, a 5 seat Skymaster has very little baggage space - a 210 has a separate baggage area.
Later Skymasters have 150 gallon tanks standard - this was an option on the earlier models.
Both have similar useful loads. The Skymaster burns more fuel. This means that for same length flight, the 210 will have a better payload (since less weight is used by the required fuel).
Fuel starvation accidents in 210 are a problem. They run out of fuel about 10 times more frequently than comparable aircraft. One theory is that since the wing has such a small dihedral, leaving the fuel level down even 1" is several gallons of fuel. Maybe people think they have 87 gallons when only 80 is on board. In a 210 you must personally check the fuel level if you're counting on full tanks (and this is a smart idea in all aircraft).
Later model 210s have a “Both” position for the fuel tanks. Earlier models had only left & right.
I’m not sure about the P210 models, but for non P models, dual alternators and vacuum pumps became standard in 1982.
P210s are optionally certified for known ice, Skymasters are not, although they are available with boots and all of the deice equipment. This shouldn’t be a big consideration, since no light aircraft (certified or not) should be poking around in known ice.
For me the deciding factor was that Skymasters have 2 engines with centerline thrust, which translates into very safe controllability with an engine out.