On a log pile I say a shorter bar is safer. If you don't need the length then all it is doing is sticking out the back side of the log you're cutting & getting into something you don't mean to be cutting (another log, ground...).
Vote for a 16" bar & add a 20" if/when you need it.
BTW, I find felling wedges very helpful for stopping pinches when bucking from a pile. Everything I scrounge is on the ground so bucking is all I've used my wedges for. Not sure if they'd be any use on logs under 10-12" though.
Oh, and your Stihl dealer sucks man