[David Dixon]

Well I suppose it's a bit difficult comparing the two since I've not owned a 337. The obvious differences are:
The 336 is slower, The aspect is not as good (horizon is high) less things to go wrong with the fixed undercarriage, rear engine appears to run hotter. noise or vibration is no problem for us,
A new rear prop was hard to find.
Make sure your props are current, we got stuck with the old out of date props that didn't have the red dye. These are illegal and it was an expensive exercise.
I wouldn't be buying a plane of this vintage without thoroughly checking for internal corrosion. Does the rear cowl operate? these cowl motors are known to give trouble. If it works that is a plus. Ours is always open as I havent sourced a rear cowl motor. In the event of a rear engine failure, the rear cowl if stuck open can cause drag. One important thing to do before any purchase is to thoroughly check the rear exhaust pipes. They have been known to fail which can cause a fire. This happened to me and the previous owner. (we now have had new stainless pipes made with minimum welding) We feel that we have overcome the problem by a thorough check of all welds at least every 50 hrs. Check if the exhaust brackets are free to move also.
Are all the seals around the engine in place? these are for the upper air to be forced through the oil cooler. There are to be no gaps for the air to escape
Test fly the plane on a hot day and monitor the rear oil temp.
If the aircraft is highly maintained and all is up to date and the price is right, go for it. We love our 336 (the only one operating in Australia). There is a fellow 336 owner in the US who is expecting to fly his 336 to Australia and is negotiating long range tanks.
I'll leave you with one thought, you're best to look at a well maintained 336 than a clapped out 337. Any more questions please feel free to ask
Regards
David