Lots of good responses to your original question. I don't know if I'm qualified to give you an answer since 1) I haven't used a 3120 to mill with and 2) I tend to look at things from an outlaw point of view most of the time.
But I will give your some of my experiences on the 395 and tell you a little about my dilema.
I have a 395 that I mill with. I run a 42 inch bar on it. When it's on my GB mill that gives me about 36 inches of cut. That 36 inches is all the 395 wants when it is buried in oak. You have to have a very sharp chain and feed the saw very uniformly at that full width capacity to not be overloading it in oak. In cherry or other softer woods not a problem. ( I find oaks such as red oak or white oak or live oak to be hard on chains and harder to cut than other species.) BTW, I don't run the "milling" chain much any more, I've found Stihl 36 RSC cuts just as well and is simpler to come up with, stays sharper longer and cuts easier. IMHO.
So If you think you will be running right up to the very maximum capacity of your mill a lot I would go for the 3120
IF the price is right. If your are only occasionally going to be at that max capacity then then 395 will do what you need to do IMHO. If you seriously think you will be needing to go to a larger bar than the 42, then definitely look to the 3120.
I always try to look at value and utility when I have one of these situations. (I have alot of these same kind of decisions to make every day on the farm and at work.) I'm always looking at the utility of a purchase and the cost and what the "return" will be.
FWIW, I went thru this same scenario a few months ago and decided to go with a double powerhead setup on my 66 inch bar. I am pleased with that approach. It works well and best of all for a skinflint like me I didn't have to buy another saw.
I run my 395 on one end and my 385 on the other end.
I felt like the 3120 didn't have as much utility for my other saw duties and the 385 and 395 handle those other chores well, so it worked for me. YMMV.
BTW, It's your money and your decision so you have to decide what will work for you and
satisfy you. I had an urge to go the double powerhead route from the very beginning and had a lot of people tell me it would not work. That was all I needed. I find it pretty satisfying to work out a unique approach for solving a problem, especially when someone says I can't do it.
Scott