Long time lurker here, decided to finally register an account as I bought a 55JS8500 last week. It was scheduled to be in stock on the 19th, with a delivery date of today, but the store I bought it from still has yet to receive their units. Was told a date of April 1st.
As for how much I paid, I got 10% off the price of $3,000. However, since I bought it locally it ended up being similar to original price after taxes. Still a good deal in my opinion.
I'd definitely go with the JS8500 over the 8550 simply because of the 10 bit panel, HDR, and quantum dots. That was what I took away from my research. Although upgrades to the 8550 are supposed to significantly improve the set. It really comes down to if you want to wait or not.