6 cores, 6 threads and 4 core / 8 threads are about equivalent. Honestly I think for future 8 threads is better than 6 (even if the 6 are real cores). We've already seen this when looking at games that need '4 cores' to run - they will load up and work (albeit slowly) on old i3's with hyper-threading, but won't even start on much newer higher clocked dual core cpu's like the G4560 as the thread count requirement is a hard limit of the game engine.
I can see 8 threads becoming the minimum in the not so distant future, this is why I never liked Intels 6 core 6 thread i5's - they are going to become obsolete imo.
Edit: for rendering performance I've just looked up the V-Ray numbers for all 3 options:
3300X: 7321
3500X: 7477
2600: 9491
So for rendering workloads, the 3300X and 3500X are on par, whilst the 2600 is 27% faster. When it comes to games, it does depend what gpu and graphics card you pair it with, and what settings and games you want to play.
If you are looking mainly at high - ultra settings with a 60fps goal in mind, then all 3 cpu's will perform about the same. If you want to get into fast paced shooters (e.g. CS:GO), play on low settings and hit high frame rates then the 3000 series parts are going to give quite a bit better results than a 2600 thanks to the stronger single thread. The two Ryzen 3000 cpu's are also less likely to bottleneck a faster gpu, provided they have enough threads for the game.
This is where it gets trickier, as the 6 cores / 12 threads of the 2600 are more likely to be 'good enough' for longer than either of the other two options as games are tending to use more cores. It's like when the first gen Ryzen 1000 series came out - at the time reviewers all recommended the 4 core 4 thread i5's over the Ryzen 5's as they proved better in games at the time. There are now many games that struggle to run on the 4 threads of the i5, whilst the 6 core 12 thread Ryzen 5 1600 still run the games with no problem. The real issue is predicting how quickly games will need the extra threads.
One thing that does occur to me - with the next gen consoles both confirmed to have 8 core, 16 thread processors, I think that might become the optimal cpu setup fairly quickly once the consoles are released as many big budget games are developed console first. If that's the case, then it might make sense to get whatever the cheapest option is for now, save up and get an 8 core Ryzen cpu next year.