Hex head and adapters makes it very versatile. It is my go to tool.


I have both the M12 and M18 hex 1/4" impacts from Milwaukee. I only work on cars and I've never had an issue with length or otherwise trying to fit them into tight spaces. I like the versatility of the hex drive.
Being able to use different adapters has been great. I mainly use the M12 with a 1/4" socket adapter for small under hood work (hose clamps, air boxes, etc.) and under car panels/splash shields. I use the M18 with a 3/8" socket adapter for anything that requires more torque including brakes and metal skid plates like those you find under some Toyota pickups and Jeeps, for example.
In the past I've also used the M18 with a 1/2" socket adapter for running down lug nuts (after starting them by hand and before final torque with a torque wrench). But the 1/2" socket adapter won't last long, so I don't do that anymore. Unfortunately it doesn't have enough juice from my experience to remove lug nuts, and the socket adapters break anyway.
Those were the first 2 Milwaukee tools I picked up, because of their versatility. Later I added the 3/8" ratchet and the 3/8" anvil head impact to my arsenal. I also plan to buy the 1/2" anvil head impact this weekend, as I already have a 6.0ah battery for it and I no longer work in a shop (sold my pneumatic impacts).
They each have their uses.

Fair enough. I should probably pick up the 1/4" anvil head myself, it would go well with my 1/4" impact sockets. But honestly, if I'm going that route, I would take the M12 over the M18. You don't need much power for 1/4" and in your scenario a smaller tool size overall would be more beneficial anyway.
Your experience may vary of course... But like I said, I've never had an issue with in terms of tool size. I guess that's usually because I'm coming at the bolt/nut/fastener from above or underneath it. I can't recall an instance I would of put these sort of tools in between 2 objects (say the radiator and front of the motor for example).
When I was building autonomous cars we used Bosch 1/4" hex impacts (similar in size to the Milwaukee M18) with adapters all day long, for both exterior and interior. Again, never found myself restricted due to size, that I can recall.
By the way, I assume you meant 18v and not 20v, correct?
