Hello,
In my point of view, the key for taking this decision ought to be about the End of Life and Fix Pack releases and, lastly, features allowed. I rather less features as possible than a robust AppServer.
Speaking of 8.5.5, the last Fix Pack was 23 December 2016 and in 04 August 2017 is planned to be released that other APAR.
Speaking of 9, the last Fix Pack was 14 March 2017 and in 23 June 2017 is planned to be released that other APAR.
So, it is up to you.