I think of Substack as the Ethereum of newsletter platforms

And Beehiiv is Solana 

An established network that is low risk to join vs. a faster moving one that is more experimental 

Bullish on both

If Substack is Ethereum and Beehiiv is Solana, Paragraph is Base

cma.xyz. Founder /paragraph, previously at Google and Coinbase

I've been conditioned to build on Base, so this checks out. 

Will also say that I've deliberated the pros and cons of each platform to the point where I just spun my wheels and didn't publish my writing. 
When I saw Paragraph it instantly clicked, and the metal block was removed.