By infrared gas I an assuming you are comparing the "orange flame" ceramic heating elements versus the "blue flame" ceramic heating elements. Both are typically open flame it is just whether or not the ceramic element is heated to glowing so as to add IR heating to the equation.
Both will provide convective heat, but if his shop is a largely open space then the infrared heater will "feel" warmer long before the shop actually warms up (at least with regard to whatever portion of your body us facing the IR heater). Either will introduce added moisture to the air, which us not necessarily a bad thing in the enter months, but something to be aware of. Each will need a reliably clear safety zone above, below, to the sides and in front of to avoid fire risk. Most importantly, neither is compatible with flammable vapors and can potentially ignite any such vapors if present in the garage. Partly for the reason they are typically installed a few feet above the floor to help reduce exposure to low lying (heavier than air) vapors. This includes spilled gasoline, spray propellants, oil-based finishes, etc.
