At 5ft 9in and wanting an upright riding position I think you need a medium or even a small,definitely not a large size.I'm 6ft tall and ride a medium size and whilst I could've bought a large size the medium has been fine the 41/2years I've been riding it.
Note,the newer model based on 27.5in wheels is the same frame as my 29er based machine,just msaller wheel diameter with bigger tyres.
I bought the Longitude because I specifically wanted to avoid mechanical suspension other than the LT sus post to take the big hits out of descending on a rough fast trail,(I have it set up with a 9 and 8 suspension rubbers so it doesnt affect my pedalling but takes care of rhe big hits).
The key to ride comfort with large section tyres is critical tyre pressures,eg; I ride mine,running very supple lighweight(760g)2.4inch Conti XKings @ 15psi front and 25psi rear with an all up weight of bike+day luggage+me of 100kg.I find anything higher makes the bike bounce around too much leading to sketchy handling on rough tracks with no increase in speed,the bike just becomes more uncomfortable leading to more fatigue andless enjoyable ride,and more unpredictable handling.
With the bigger 2.8 or 3inch tyres on the current model there's scope for reducing those pressures a lot more especially if you're a lightweight.
Interesting video interview on tyre pressures/speed/comfort,whilst not directly related to MTB's the principles are just the same:- https://youtu.be/asEYkpW0vwQ