A key difference is the ease with nostr that you can take your keys and thus your notes to other relays. I tried pretty hard to use mastadon and it was really hard to find people and interact with people on other servers. My understanding too was that if you joined a certain server and got banned by the server owner, you'd be pretty bummed because you'd need to start all over with making a new account, friending people again, finding them, etc (might not even be able to find them if they are all on that server you got banned from).
Nostr makes it really easy to pack up your things and leave a relay in comparison, and find your friends on other mutually shared relays if you got banned. This reality that people can easily move to the best relays will reduce censorship attempts, especially as relays try and monetize (in my opinion).
To me, it's kind of like the diff between ethereum staking and bitcoin mining. Ethereum staking is highly centralized, and if u have your ETH in a staked pool like coinbase, you can't move that stake you are completely captured and stuck. But with bitcoin mining, even though there are 3 pools responsible for generating more than 50% of the hash, it's just not the same because the people with the ASICs can point those miners at a different pool with a few clicks of a button.