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

The world's best cartridge will not sound great with the world's worst phono stage but budget cartridges will benefit from the world's best phono stage. Best to buy the best performing phono stage you can afford and work your way up to a cartridge which equals the the phono stage.

Before cartridge and phono stage get the best turntable and arm you can afford. Buy second hand if prices are intimidating,

That's generally my thinking as well.

Get everything possible out of your excellent cartridge and only then upgrade it.

Similar with speakers.

I had a Goldring E3 mounted on a Rega P2 and was using a very decent phono stage built in with Rogue RP1.  Sounded great. Then I tried the Mcintosh MP100 phono stage I borrowed from my buddy and my jaw dropped. I paid $100 for that Goldring. It sounded insanely good. I had to return the MP100 and continued using the built in phono. Later on I stepped up to Hana EH cartridge and while the upgrade was very audible, it was nowhere near the level of improvement brought by the upgraded phono stage. 

@inna 

dwette, could you post what you upgraded to what ? And what turntable/tonearms do you have

1. Clearaudio Ovation/Universal with DV XV-1s and Naim Superline phono: upgraded to Lyra Atlas cart
2. Upgraded to Clearaudio Innovation TT with two 12" Universal arms, added Lyra Atlas Mono
3. 18 months later, upgraded Superline to Boulder 1108
4. I have since upgraded Innovation to Master Innovation, and added 3rd Tracer arm with my previous DV XV-1s

"Let's assume already pretty high level of both."

Alignment Tools and Skills: acquire the few inexpensive tools and learn to do it yourself.

Go beyond: Think long, think about options, flexibility, it took me over 40 years, retirement, and rethinking everything during covid, and a lot of help here for this.

The great unknown, who knew my friend would leave me his collection of over 4,000 LPs, mostly Jazz, many played only once.

Stereo/Mono/MM/LOMC/MI, I'm a big fan of two tonearms, Stereo and Mono aligned, calibrated ready to play in an instant, combined with a small collection of cartridges pre-mounted in headshells with azimuth adjustment, and easy adjustable arm height.

If you have one darn good cartridge, with an advanced stylus shape, even on basic aluminum, it is good enough to find and settle on a MM Phono stage. Get that DONE.

to me, my Tube MM Phono is done for life, as is my SUT, to pre-boost LOMC so I can keep using it.

I have added other new cartridges, taken risks with used Vintage otherwise unobtainable cantilever materials, and enjoy listening to friend's cartridges they bring here.

We all have Steve and Ray Leung at VAS and Peter at Soundsmith to expand the possibilities, including purposely buying broken cartridges.