A bundle is indeed a combination deal (mobo+CPU) offered by a retailer. It can sometimes save money.
For me, the 3600 is a no-brainer. It's the same clock speed and (former) price as the 2600X, but uses 30 watts less power. (Less expense, less heat, less environmental damage.) It also turns out to have improved performance. I don't game at all, but I suspect that improved performance would be significant for gaming, especially if you get 3200MHz DRAM. But the 2600X isn't bad at all, and it might save you a lot of money.
You might also consider the longer term. I have neither the money nor the interest in frequently updating my computer, so I'm looking for something that will last for years. My current computer is 9 years old. I think I did a pretty good job of future-proofing when I built it in 2010. The latest version of Tom's HW article about CPU tiers shows its Intel i7-860 as "still viable." I need to replace it because Microsoft will pull the plug on Windows 7 in January, and my mobo is not compatible with Windows 10. (It's also 9 years old. I just replaced the video card-- I'll use it in the new build-- and other components may be at the end or their lives.) Future-proofing is another argument for going with the X570. Maybe the price of M.2 SSD will decline enough make replacing the hard drive for secondary storage practical, and the X570 can accommodate a second M.2 drive without mucking up SATA or PCIe bandwidth.
That said, only you can make the right decision. The 3600 is clearly better, though I'm increasingly convinced the X570 is the way to go for it. That will cost more, though it may be worth the extra money in the long term. But I don't think you could go too far wrong with a 2600X and a B450 motherboard, especially if you get a good price on the "legacy" CPU.