What do you usually upgrade first - cartridge or phono stage ?

this 3 lp set contains mostly Stereo, however side one, the 1st 3 tracks are Mono from 1939 and 1956, I play them with my left Mono arm, SUT input C, then I switch to either of my two Stereo arms for track 4 and all of discs 2 and 3

https://www.discogs.com/release/2003328-Various-The-Bass

There is a lot of Bob Thiele and Rudy Van Gelder here

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Late invite, I snatched a few LPs from downstairs to give to a friend for a birthday present, selected the artists, but failed to notice, they were Mono LPs. He has heard the difference here, so now he is on the hunt for a Mono Cartridge.  

If you are playing Mono LPs with a Stereo Cartridge, Mono MODE, that's what I did for most of my life, until I decided to trust what I read here, and tried True Mono. I am so glad I tried, it's not night and day different, but there is a substantial elimination of noise potential from any vertical movement, surface scuffs,.... and an increase in distinction of individual instruments, voices, the improvement varies from a little to a whole lot.

I trusted, trust me, my 1st Mono, Grado Mono ME+, only $185, elliptical on aluminum is a winner. Replaceable stylus $100. Tracks at only 1.5 gm

https://gradolabs.com/products/grado-prestige-mono-phono-cartridge-model-me

I have never heard a Cartridge create the same impression that a Phonostage can. Cartridges are having their full potential removed when not replayed through a carefully  thought out circuitry.

OP, I think the answer is going to be different for everyone. For me, after I've lived with a component for years, and along the way, made changes....you kind of get a feeling which direction you need to go 

Phono stage. As my systems got better over almost three decades I was shocked how every time I upgraded the  phono stage how much better the sound go... more so than other components. Not until I got to an Audio Research PH9 did the improvement gain slow. 

@inna I am blaming you and this thread for potentially expediting my phono stage upgrade. Lol