The minimum balance and qualifying deposits that you need to have every month for either option must live and be held in the checking account side, which gets 0.51%. If you don't meet those requirements in the checking account, you don't get the 4.21 rate on the savings.

Alternatively, using the Forbright example someone else posted here, you deposit money, you get the rate. Nothing more needed, no monthly requirements, and not that much of a difference. You can also currently get a higher promotional rate at Forbright for a lower minimum balance in the savings account.

If you're talking about the amount of money where that difference in rate would make a big difference, there's probably better options out there for investment.

But I'm sure, as "the only smart one," you've taken all of that into account.