C320 was a turbo charged version of the C310. Then Cessna made all the series turbo charged which made the C320 redundant and it went out of production.
There were differences in the windows and also in the position of the rear bulkhead which changed the available cabin volume a little, but usually the changes were available in either model.
The external dimensions (span, length etc) were the same for each model.
I fly both at present and even though they are identical in external size, it is easy to be fooled when looking at the cabin. One has a ski tube but has the bulkhead further forward, one has a higher cabin roof over the pilot station, one has a two piece windscreen etc. One has a takeoff weight of 5670lb and the other is 5300 lb, but the useful load is about the same.
What I like about the C320 is that it has 285hp engines and is very quiet. It flies to about 25,000 feet and burns 32 gph while the C310 has 325 hp RAMs and can go higher (30,000 feet?) burning 36 gph but is so noisy it drives me mad after a few hours, even with a noise-cancelling headset.
Both are really fun to fly, and can be operated out of 3000 feet gravel strips safely, with good single engine performance if required. If you don't worry about engine failure, they can fly out of 2000 feet strips, although I usually use flap then, and pull them into the air early to avoid prop damage from rocks.
And if you have flown either, you already know the cabin heaters will have stopped working long before you reach 25,000 feet.
Parts are hard to get and expensive. We have not had the spars done, but I think all the ones in Aus have been modified?
If you get a chance to fly one, go for it. I guarantee you will love it.