My personal experience with the 391 is that they're generally underwhelming; a little big in the britches and a little doggy in the power department. Now, I do believe Stihl has made some great progress in the non-pro-saw construction when it comes to these, so I'm not saying they're a dud.
I have a 390 and with a muffler modd its capable of easily cutting 18" oak. Its all the saw wants to burry a 20" bar though. Forget a 25" bar unless you just have to use it. You won't win a race bit get it done. I did for years. What are you useing now?
If you're looking to run an 18"/20" bar and want to buy a new saw I would focus more on the MS291 or MS311 and save some coin. All things considered I've heard very good things about the MS291 and I think it might just fit your needs quite well. Then, you can use that extra jingle to buy a case, chains, file, mix oil, bar oil, etc.
How much $ is a 391? It's miserable here, 23° and the snow we're supposedly getting is freezing rain & sleet.
Prices may vary, but here in the great state of Indiana full retail on an MS391 with a 20" bar is $619.95.
Are you dead set on a new saw? I don't mean to turn this into a classifieds ad, but I have a nice MS310 that I just put a new top end in and a nice MS361 that just got a new piston. Are you wanting a warranty? Don't get me wrong, good dealer support is a huge advantage and not to be taken lightly.
