There has been a lot of good advice so far. My two cents.
I have a 150mm f/8 frac and like it, though it is a beast. After coming from newts the colour on the frac is noticeable. A Baader Fringekiller helps but there is still a bit of blue on the lunar limb. I expect there to be more false colour in an f/5. A 45 degree diagonal is useless for astronomy. You will need a good 2" diagonal.
Achieving good focus with the stock focusser was always difficult due to tube shake. The shorter tube will be an advantage there but on the other hand my mount is more solid. Initially I added a motor drive to the stock focusser but after after a few years gave up and upgraded to a motorised Moonlite. New scope! On the other hand, the more rigid mounting on my 250mm dobs means that the stock focusser is fine.
A 150mm scope gathers 1.5625x more light than a 120mm scope. Allowing for the secondary and slightly greater light loss you would still have >1.4x times as much light with the newt. However the generally better baffling of the frac vs the stock newt will narrow that gap - especially around light pollution. Of course improving the baffling of the newt is easy and cheap (says he who hasn't done it yet
).
You need to learn to clean and collimate newts. Neither task is difficult but it is a skill you need to learn. Solid tube newts will hold collimation if just moved about the house but after a road trip they generally need a bit of adjustment. After a few years a newt will need re-aluminising, which will cost a few hundred. If you then get an overcoat added you can expect at least a decade before it needs to be done again. Well, that's what I'm hoping - mine was done last year.
Happy decision making! But whatever you choose, just enjoy the scope.