Investments are chiefly Fidelity index funds, with premium and institutional class offerings, although the platform will also offer some BlackRock exchange-traded funds. While both retirement and taxable accounts include Fidelity index funds, taxable accounts can also include BlackRock iShares exchange-traded funds and tax-advantaged municipal bond funds. Both account types typically hold less than 1 percent of assets in money market funds.

Accounts have a $5,000 minimum, and Fidelity says the "[c]urrent all-in costs, including the advisory fee and underlying fund fees, range from 0.35-0.40 percent, which may vary over time based on changes in the underlying investments." That cost will start "at approximately $20 a year," Rich Compson, head of managed accounts at Fidelity, said in a statement.

Robo-advisors Betterment and Wealthfront structure their fees differently. Wealthfront charges 25 basis points a year on assets over $10,000, with accounts under $10,000 not charged an advisory fee. Fees for the exchange-traded funds it offers average 0.12 percent, according to its website.

Betterment's website quotes its management fees as ranging between 0.15-0.35 percent, depending on balance and whether a client has auto deposit. Accounts under $10,000 are charged 0.35 percent (with auto deposits of at least $100 a month), or $3 per month without auto deposit. Accounts over $10,000 have no deposit requirements and the fee falls to 0.25 percent; accounts over $100,000 fall to 0.15 percent.

Fidelity Go is also integrated with the firm's online financial planning tools, and provides users with ongoing monitoring via Fidelity mobile apps, including Apple Watch alerts. One more attraction the firm offers is the ability to direct rewards from the firm's Visa card into Fidelity Go accounts.