Mastodon is substantially more active than Lemmy thus far. It’s a different type of medium to Lemmy, so what you want to use is up to you.
Personally I use both, but just like I was spending much more time on reddit than twitter, I’d like to spend more time on Lemmy than Mastodon. I have also found Lemmy a little more intuitive to use initially, so it might be worth spending a couple weeks getting used to how the fediverse works before jumping into Mastodon.
None. These are very much enterprise-oriented updates. These extreme speeds are so difficult to achieve that it outweighs the simplification of scaling down the number of lanes. It’s better to use a PCIe 5.0 x16 connection than PCIe 7.0 x4 in consumer hardware.