I don't know the reason why some brokers has the BTC/USD pair and others don't but I think it might be due to regulatory hurdle. Some of this exchanges don't have the jurisdictions to open up their services to US users. I think that's why Binance doesn't have USD pairs but Binance.us should have it (not sure). For this reason, exchanges have to look for alternatives - stablecoin projects like USDT and USDC since they're are pegged to USD.

That said, they'd all work just fine.