I do some milling.
I started out with a 272xp and gave up on it pretty quickly. I bought a rebuilt 066 and then added a WP BB kit and have milled with it (660 as a backup) mostly since 2008. Recently I decided that the Stihl was lacking in 30"+ wood (milling) so decided to use my 3120xp...bought a 394xp as the backup saw(for bar interchangeability) , and have only used the 394xp!
(1) The 394xp's oiler is at least adequate of the 42" bar, and the Stihl's seems lacking on even a 32". There is an oiler upgrade kit, or HV oil pump available for the 066/660. I did the kit and am still not thrilled.:msp_unsure:
(2) The 394xp's air filtration, and mine is the low top, is simply amazing!:msp_w00t: The 394 attracts almost no sawdust as compared to the Stihl which acts like a vacuum cleaner. The Stihl HD air cleaner is good, but needs to be checked frequently.
(3) AV? I guess that I'm not terribly sensitive here, but the Husqvarna spring AV is noticeably smoother. There are 2 or 3 versions of Stihl's rubber AV mounts from soft to firm that can be changed to suit the user.
(4) I found my used 394xp considerably stronger/holds more RPM in the cut than does my 066BB, but talking about milling 30"+ wood, so smaller wood may not be noticeable, and particularly if soft wood.
(5) The 066/660 is about 1# lighter, but not an issue on the mill. Realistically, the difference between a 16.5# powerhead and a 17.5# powerhead isn't noticeable, and particularly when you add 28"+ B&C.
(6) The 066/660 has the side chain adjuster which is far more convenient.
I modified my Granberg Alaskan to allow easier adjustment access for the Husqvarnas.
(7) the 066/660 has the outboard clutch which makes changing B&C simpler, BUT.....inboard clutch puts more load on the crank further from the bearing, so is a weaker point. It also introduces more heat into the crank case, so is more apt to overheat a seal. My only failure in this regard has been a clutch when the carrier/spider BROKE. Luckily it didn't damage the threads on the crank.
There is definitely an advantage to having a dedicated milling saw in addition to your felling/bucking saw. It is also advantageous to buy a used saw that has decent parts availability. There are lots of 066/660 and 394/395xps out there, and lots of parts available, so would be my suggestion of saws for milling without resorting to the 7 cube class.