its important to find the correct balance, but at the beginning for a new player its definitely more rewarding to upgrade runes
It's difficult. A lot depends on how you want to play. Do you wanna have just some fun times in the game with no stress?
Then forget building up runes, or invest in any CS item; the time to farm these is way too high and the fun you wanna have wouldn't exist for at least several months until you got the 'correct balance'.
If you wanna get into endcontent, then you'll need at least the first two CS-items of every kind (earring-slot, carnifex, stat-rolls, etc etc) and full 6 - 7er Runes, in my personal opinion. Farming for that would take a lot of time and would get you under some strain, so I just recommend you to invest some money to the game to preclude that. I
f you reach these conditions, you can finally enjoy the endcontent of the game in the end.
Whether this is it worth or not, you must think for you own.
I can just tell you, that this game has some really good aspects and a gameplay which holds me playing it for nearly two years now. The only problem here is, that you need some requirements to reach them, and these are the runes and CS-item in general, which I mentioned above.