I'm looking at two different conversion vans. The first is a 99 with 40k miles and the second a 89 with 20k miles. Both vehicles check out on Carfax. The 99 is a Dodge Ram and the other is a 89 is a Ford Econoline. I like the looks of the 89 much better, and it's been garage kept so the paint is like new but not so much for the 99. My question is, what factor is more important? the age, or the mileage of of the vehicle (all other things being equal ..).
Since you're asking specifically about a conversion van, and the mileage on both is relatively low, I'd say in this case age is going to be the most important.
There are not enough miles on either van to cause any significant mechanical issues. Age on the other hand will "wear out" rubber components, electronics, and most importantly for a van it will have an affect on the interior. Cushioning will break down, there are 10 more years for moisture to be an issue or bugs, or mold, or whatever.
That said, it really comes down to inspecting each carefully. Pay attention to the rubber on the 89 and the interior. Just because they tried to keep it pristine, there are just some things you can stop from deteriorating.
I base my decisions on several factors when purchasing used vehicles. I research the history of the vehicle to see if at any mileage interval the vehicle has reported issues.
Another issue would be if you can even get parts at the parts store or would you have to go to a junk yard.
Last, what are the insurance premiums on either vehicle based on the mileage.
Either way you look at it the answers you will get will be personal preference. There are however other factors to keep in mind that you do not mention such as body.
So to answer your question:
What is more important the age or the mileage of a vehicle
Depends on what the research for the vehicle determines. Some vehicles at certain mileage intervals may have reported mechanical issues. If the vehicle was garage kept the chances of rust should be slimmer.
I've never known all other things be equal in two vehicles of such disparate ages.
In such cases I'd normally look at secondary matters like the features.
Do they both have ABS/disc brakes all round.
Is one of them a better spec than the other.
What is the fuel consumption of each vehicle.
Do they both have full airbags.
You'll almost certainly be better off with the more modern vehicle unless you're a real fan of the older one. Most vehicles were still quite primitive in '89.
This is one of the things that's more considered in Europe than the US but there are a few reasons not to buy a vehicle from before the 90s.
Leaded fuel: This was not actually phased out until 1992 for new vehicles.
Emissions regulations: Especially as this is a van, you wouldn't be allowed to take it into a lot of cities. To put that in context, standard is currently the Euro5 diesel engine, by 2020 this will not be allowed into some low emissions zones.
Safety: ABS has been required since 2007 but was standard on most vehicles since about the mid-90s. Airbags were also introduced as standard sometime in the mid to late 90s. Side impact protection is later but also standard on all modern vehicles. General impact protect also massively improved in this period.
Security: If you lost your key to a car from before the mid-90s you could walk into a dealership and ask for a new one which they'd give you off the shelf. If you had a Vauxhall (Opel) Cavalier, it didn't matter, any key for that model would work.