I Bonds combine a fixed rate with inflation adjustments, but they also have a $10,000 electronic annual purchase limit, a 12-month lock, and an early-redemption penalty before five years.
Fidelity CMA is a brokerage account with transaction features and two cash-core choices, while a high-yield savings account is usually a direct bank deposit designed primarily for saving.
Betterment Cash Reserve uses a brokerage-based Program Bank sweep, while Betterment Checking is provided through nbkc bank and is designed for everyday spending.