I have a 661 and a 394, and my opinion is varied. The 394 has a slight edge on power, (slight) and it's the better saw for milling without a doubt, but I use it almost never because I hate the way it handles.
My 394 feels >5lbs heavier than it really is to me after a full day of use.
I just got the 661 a year and change ago, but have run over 35 gallons through it, and find myself choosing it over all other saws ( 20+ saws in 55-94cc) I own including for small stuff.
I don't have comparative numbers, but using after a full day I come out tired and disliking the 394. for felling, it's a boat anchor. If a tree fell on it I would not cry, (which is why that guy probably swapped saws. lmao) 394/95 is a Very well built saw, and I chalk this handling data up to my personal preference. yours may differ.
after using the 661, I am still tired, but don't hate the saw, and I gladly pick it up again and again. My 661 puts a smile on my face everytime
661 is significantly less fuel thirsty, but both these saws drink fuel pretty fast and I don't consider this a driving factor in the choice.
394 has a better oiler, especially on max inch bars.
With a 36" bar wearing full comp burried in oak the 394 is a full second faster in my tests, but the 661 motor temp is cooler after the cut.