Both the X320 and 2550 are excellent machines. The 2550 clearly provides more machine for roughly the same money with its GT frame, shaft drive, and slightly larger deck. However, after owning at GT2554 I believe the JD has better ergonomics, if that is an issue for you. Both the Kohler and Kawasaki engines are excellent. I personally prefer the JD hydro pedal set-up to Cub Cadet's pedal layout; for one, I found the CC reverse pedal somewhat awkward to use compared to Deere's side-by-side pedals, and for another, under certain circumstances I discovered that the mower deck on my 2554 interfered with the reverse pedal.
If you NEED a garden tractor, then the 2550 wins hands down. The 2550 will be far superior to the Deere if you plan to pull heavy loads, ground engaging attachments, or work on very steep terrain. Otherwise, if you simply want a machine to mow the yard in comfort, apply lime, fertilizer, and seed occassionally, and aerate with a normal box-store quality ( approx 300# with ballast ) aerator a couple times per year, then you may find the X320 to be a more comfortable and satisfying machine to operate. However, there is also a very real sensation that you're getting less machine with the X320--but you get a little more comfort and user friendliness. Also, though the CC is shaft drive and shaft drive is nicer/stronger/more expensive than a belt driven trans., I wouldn't be at all afraid of the durability/serviceability of JD's belt drive as the belt driven X-series mowers are some of the best belt drive mowers on the market. One last point on the 2550--I love CC's mow in reverse feature. You simply turn the key back one click after starting the mower and press a button on the dash and you can mow back and forth all day until you shut the engine off without doing anything else.
To answer your specific questions: 1) when I was shopping CC dealers stuck to the msrp shown on CC's website for pricing, so 3699 is basically 100 off and is a good price 2) the 2550 is built with very solid components but QC may be lacking at the factory so you may have some issues to sort out after delivery, but the engine, trans, frame and deck are all built like a tank to take hard work and abuse 3) You may be satisfied, I wasn't with my 2554. If you simply want the absolute strongest, heaviest, most heavy duty, largest GT you can get for your money then barring catastrophic failure you will be satisfied with the 2550; if ergonomics and the small details carry matter to you in the long run then you may not be satisfied after the new wears off--or you may be, satisfaction is hard to predict. There are a lot of satisfied 2500 series owners 4) yes it will definately last--if you wait for it to wear out before you buy a new mower, it will probably last far longer than you want it to
