Regarding the M12 vs M18 compact for car work, I would say get the M18 compact, I use one daily and it's just about adequate; while I don't own an M12 I really wouldn't want anything less than the M18.
-Power on the M18 drops off precipitously with extensions/attachments, there have been times it couldn't loosen suspension bolts that were only accessible via extension, and any type of universal joint is also killer.
-It takes off reasonably torqued (~80 ft lbs) lug nuts within a couple impacts (<1 second), it struggles a bit (3-4 seconds) at anything notably higher but I haven't encountered a lug nut it hasn't been able to eventually loosen (worst I've encountered was maybe 7 seconds); this is with no extra attachments. The exceptions are wheel locks, the addition of the key and the poor impact transfer means that moderately overtightened wheel locks are a struggle, 10+ seconds sometimes, anything more, the M18 is not be able to remove.
-The M18's power starts dropping noticeably when the battery is at around half charge (I have XC 6.0 packs). The tool still feels powerful so it might be hard to tell, and it's not anywhere near as bad as with NiCads, but it'll start taking longer and longer to loosen things, to the point where I always swap batteries at half charge now. If something similar happens to the M12 it could start pushing it out of usable territory for lug nuts (and it will reach this point much sooner than on the M18).
-The big issue for me: these smaller rapid hitting electric impacts are loud, and any time it struggles you are risking damaging your hearing and the longer you are exposed to it the worse it is. It gets particularly bad in certain situations, like with steel wheels or monkeying around in a wheel well/enclosed space, in those conditions 3 or 4 seconds is a long and painful time, I always wear hearing protection.
There's also the issue of power loss as your batteries age or operate in sub optimal conditions such as the cold, or the tool just wearing out and having more drag. In my opinion the M18 has just enough extra power to create a reasonable but not particularly amazing buffer against this sort of thing.