Buying new will eliminate the ongoing maintenance required with busted arse old PA-31's and SIDs for Cessnas and you will still have a decent retained value if an upgrade is required after a few years. But seriously, how many people are needing to be carted around at the moment?
The new aircraft still require much the same maintenance an old one does. Operated 30 year old Seneca II/III and brand new Seneca IV and V models and the 30 year old aircraft were more reliable, used less fuel for the same speed and carried better loads. This was probably due to sorcing aircraft in good condition and history. Brand new you don't know until you operate it and the waranties are not particularly long lived. The newer aircraft were quieter and had modern features and equipment, slightly better take-off performance (due more powerful engines) but at max weights were much the same as the old. Unfortunately nothing much has changed in the way they make light twins in the last 40 years and the engines are the same possibly with electronic monitoring and control gadgets (which just add more complexity, weight and reliability issues themselves).
If you buy old just do a lot of homework and get a good one, then keep it in good condition.
As far as value is concerned generally I have sold all around what they were bought for with the older aircraft as they have already fully depreciated. Just be very careful of buying higher hour aircraft and make sure they are not approaching an airframe limit etc...
A new aircraft like a new car will lose significant value as soon as you take possesion of it.