Worth is something only YOU can determine.
From your current parts, I speculate that gaming is most important to you.
Some games are graphics limited like fast action shooters.
Others are cpu core speed limited like strategy, sims, and mmo.
Multiplayer tends to like many threads.
Is your 9600K overclocked?
What is the make/model of your case and fan arrangement?
What is the make/model of your cpu cooler?
Most games will not effectively use more than the 6 threads that you now have.
Upgrading to a 9900K would gain you some threads and single thread performance which is always good.
If you are multitasking while gaming, then definitely upgrade to the 9900K.
You can recoup some of the cost from selling the 9600K.
If you play multiplayer with many participants, then 9900K is a good upgrade.
Lastly, if you have the budget, go ahead and upgrade.
If you do not, you will forever be second guessing yourself.