Honestly, for me, it doesn't matter anyway whatever you use, and personally, i don't even pay so much attention to it when typing bitcoin or Bitcoin.
I guess the B is better or maybe
BTC at some point since this is available in the forum.
When we talk about Bitcoin as a blockchain, payment method, or a service, we are supposed to spell the word with an uppercase B.
When we talk about the currency or mention numbers and quantitates of the asset, the word should be spelt with a lowercase b.
Maybe the difference between the two is not really that important so anyone could just use Bitcoin or bitcoin in general. So in the end it is still understandable when explained here in the forum.