It’s easier to upgrade the SSD later, also a smaller SSD won’t impact gaming performance. If you get the RAM you need then you can get the better gaming experience, you just need to manage storage space.
as sizzling said, it depends on your use case. you'll gain performance primarily due to added ram, not due to the fact that you'll be running dual channel. if you play games like warzone, battlefield 1, 5 or any other modern game then another 8gb stick will help a lot
Theoretically, transfer rate is doubled, so you want dual channel whenever you can especially if the motherboard and CPU can support it.
It is the same reason to get quad channel instead of dual, there is quite a difference when you double. It's like a street with one lane versus two, two lanes can flow better.
It is true if you browse web pages or run minimal tasks there may not seem like much of a difference, but with any performance task / game then a difference should be noticeable. Run any memory benchmark test and it will show quite a jump from single to dual channel.