Clint,
since you are probably the most knowledgable guy on here regarding HVAC
what about giving us all a recommendation for a standard two car garage type of setup.
Dave jr.
Dave, I wish there was a good answer that would work for everybody. Too many variables to satisfy. However, if it was my two car garage, a gas wall furnace is hard to beat. Simple to install and vent outside (especially in a garage where the water heater is likely to be), low cost, heats very quickly, and takes up almost no space. But it does have an open flame which means no painting with it on. There is going to be upsides and downsides to every option.
Most garages are not wired with enough amps to handle any serious electric heater. Most of the off the shelf portable heaters are about 1500 watts max. That's not a lot of heat for a two car garage, even if you used two of them. But plug in more than that and you're going to start popping circut breakers or fuses. So, while there are electric systems that will work, some circut upgrades will usually be in order.
Systems that have some type of airflow fan such as through the wall or split system heat pumps would heat and cool the best, but do require constant vigilance on keeping the system clean. The fine balsa dust we create will wreak havac on these things. But if you can deal with that, the plus side is no flame, so perfectly safe when painting.
There is also the question of how fast you need the place to heat up. Can you leave the heat on all the time? Or, do you just turn it on when you go out there? Most electric systems are going to be slower than gas systems (I said "most", but not all). Of course, all this relates again to cost.
What I cannot recommend is
unvented gas systems in a closed enviroment. That includes Nat gat, LP (propane), and kerosene type heaters like the "Mr. Heater". Those are for outdoor heating in a well vented area, so don't risk it.
I cannot possibly address all the options in one post, so if there are systems you want to know about ask me.