Seeing how this has gone astray anyway, my .02.
I always recomend a saw one size bigger than what you know you need. That elminates the instance when you get something just outside of the paramaters. Like the once in a great while 25" Oak, or the 30" Pine.
The 361 gives the option of being able to run up to a 28" bar (NOT optimal) in a few instances.
The 260 will run a 20" but not happily, and yes you can drop a 20" tree with an 8" bar if you need to, but there are times where reaching across the whole tree at once has MAJOR safty advantages! And I don't care for the 260 myself anyway.
The 270 seems alright, never run a 280 so I don't know how it holds up with a 20".
The 290-310-390 series is an option, but they all weigh the same, so buy the 390 for the weight, then you are right at the price with the 361 and may regret not just going ahead and getting the 361 like I have done.
The OP asked for Stihl, I see no reason why we can't respect that.