Use impact driver or impact wrench for lag screws?
I think the answer to this will be "it depends" since there are different size lag bolts, there's the number of bolts you intend to install and how often, the size of the pilot hole you intend to drill, and you may have more or less usage for other tasks (cars) than other people.

The impact wrench will have more capacity, but may be less portable, heavier, and could over torque in some situations. The impact driver is likely to be more convenient, more likely to have interchangeable bits and batteries with your power drill and or other tools, but may not be up to driving hundreds of bolts per day. And if you're only doing a few bolts, the answer is neither and use a simple socket set to install them by hand. Regardless of which tool you use, a properly sized pilot hole will make the job easier, straighter, and less likely to split the wood.