If you're going to be using either of those for heating during cold weather then definitely the Mr. Buddy.
Butane is notorious for putting out low BTUs when the canister is cold. I actually warm my canister before cooking breakfast when it's below about 40F.
You didn't note whether you were talking 0 F or 0 C, sure hope it's Celsius cause 0 F is going to really, really suck in an uninsulated van. If nothing else, I suggest you hang moving blankets and use some reflectix in the windows to help keep the heat in.
A Mr. Buddy hooked to a 20 lb tank gets me about 9 to 10 days full time heating (NOT run during sleeping hours). My high top van is not insulated but has reflectix installed in all the walls and ceiling.
I don't find a problem with that much condensation that everyone else warns against but then I always have sufficient ventilation for the heater and am in and out or have an open window when I'm cooking.
Butane will be extremely costly as will the 1 lb propane tanks. A friend got maybe 4 - 5 hours of run time out of a 1 lb tank with the Mr. Buddy on low.
Whatever you do, DO NOT run any non-outside vented heater while you are sleeping (except electric). Choose a warm sleep system that will allow you to comfortably spend the night. There are many, many threads around here on how to match your sleeping gear to the weather.
I get out of bed in the morning, turn on the heater and then cuddle in the warm bed for 20 minutes or so until the van is warm.