IMO, a 3/8" impact in 12V with so little torque is bass ackwards. I've always used, and sworn by, my 1/4 hex impact drivers. I don't have enough problems using insert adapters to justify buying a separate tool that doesn't gain me a whole lot, and loses a lot of versatility. Oh, and according to Makita, the 12V is actually 1/4" LONGER than the 18V version, somehow.
I just put myself in a similar boat today. I've had 18V LXT for a few years, and broke my drill last week. I decided I was done with the LXT and got an M12 Fuel drill/driver and impact driver. I'll no doubt still end up with M18 for a hammerdrill, grinder, sawzall, and circ saw, but I'm thinking if I do buy an 18V impact, it'll probably be a 3/8" rather than a hex driver... not much need for the hex when I have a more compact one that still has 100 lb./ft. of torque, I'll probably be using sockets and not bits if I need more than that.