What are you mowing? I think long ago you posted you mow 3 acres-- is that still the same property? Does it have hills, slope, etc that might benefit from the locking differential in the x5XX and above series? And do you expect to use any attachments, also pushing you toward the x5XX or larger series?
My mowing needs are less-- only about a half acre, mostly flat. Although there is one sloped area we never even tried to do with our old Craftsman mower. It was too tippy and with no locking differential. I think my needs are less than yours-- more in the x3XX class, but I'm not sure. x5XX class would probably be overkill for me- I don't intend to use any of the attachments. If I did expect to use attachments, I'd skip the x3XX series and be shopping for the x5XX class minimum.
I was sold on the x380, which is a great machine, until I learned about a new Husqvarna model for 2017. The Husky GT48DXLS has a 24 hp Kawasaki engine, K66 transaxle, and electric locking differential-- for under $3k. They have bigger deck sizes but I want 48" as I will probably add a cyclone rake in the future. Fortunately I have good dealer support, as the dealer who handles the GT models is the same dealer I have bought many Stihl products from. I have not bought anything yet, but need to decide soon.