Source: Molina Healthcare.

UnitedHealth Group (NYSE:UNH) is the nation's biggest health insurer and will probably see its sales and earnings per share climb in the coming year, thanks to its decision to increase its exposure to the Affordable Care Act insurance exchanges. But it's the much smaller Molina Healthcare (NYSE:MOH) that's expected to enjoy the biggest earnings growth in 2015.

Rapidly rising
UnitedHealth is the leading health insurance provider in the U.S., serving 44.9 million members through its employer-based, individual, and government health insurance programs. Although UnitedHealth's tepid approach to the Obamacare exchanges last year weighed down membership growth, the company
still reported that its third-quarter revenue of $32.8 billion grew 7% from a year ago. Driving that revenue growth was increasing adoption of Medicare plans tied to aging America and rising Medicaid enrollment thanks to Medicaid expansion in 24 states and the District of Columbia last year. Third-quarter revenue from UnitedHealth's Medicare business improved 4% to $11.5 billion, and sales in UnitedHealth's community and state business jumped 34% to $6.1 billion.

Those are impressive numbers, given the levels of uncertainty leading up to implementation of the ACA last fall, but UnitedHealth investors have reason to expect that next year will be even better. That's because the company will continue to benefit from Medicare and Medicaid tailwinds, and it is also participating in 23 ACA health insurance exchanges this time around.

Given the potential growth for UnitedHealth in those businesses next year, it may be surprising to learn that it's Molina Healthcare, not UnitedHealth, that's expected to post the biggest earnings-per-share jump in 2015. Analysts think that UnitedHealth's earnings per share will grow by a respectable 8.7% next year, but they believe that Molina's earnings per share will surge by an eye-popping 106% to $2.64 next year from estimates of $1.28 this year.

Analyst optimism over Molina's earnings growth stems from Molina's focus on running state Medicaid programs. While UnitedHealth posted remarkable growth in its Medicaid business in the past year, the company's other businesses watered down Medicaid's impact on its overall results. That's not a problem for Molina, which gets virtually all of its sales from running state and community programs.

In the past year, Molina has benefited significantly from Medicaid expansion in its biggest markets, including California, Washington, and Ohio. Membership in California increased year over year by 363,000 members to 496,000 members last quarter. A similar increase occurred in both Washington, where membership grew by 74,000 members to 473,000 members, and Ohio, where membership climbed by 76,000 to 337,000. Across all 11 states in which Molina offers plans, total membership grew from 1.94 million people last year to 2.4 million people this past quarter.

Molina's membership growth is translating into significant revenue growth that can be leveraged against fixed costs for profit growth. The company's sales improved from $1.58 billion in the third quarter of 2013 to $2.32 billion last quarter. Importantly, the company's growth in member premium revenue has offset the rising cost of providing care to its members, leading Molina to report adjusted net income per share of $0.83 in the third quarter, up 17% from the year before.

A great year ahead?
UnitedHealth has a good shot at hitting analyst earnings growth targets next year, but given that UnitedHealth opted out of broad participation in the ACA exchanges last year, investors are right to wonder exactly how the company's decision to participate fully in the ACA exchanges will play out this year. As a result, speculative investors might find Molina's Medicaid focus more attractive, since that business offers an arguably more predictable path for revenue and profit growth.

Todd Campbell owns shares of Molina Healthcare. Todd owns E.B. Capital Markets, LLC. E.B. Capital's clients may or may not have positions in the companies mentioned. Todd owns Gundalow Advisors, LLC. Gundalow's clients do not have positions in the companies mentioned. The Motley Fool recommends UnitedHealth Group. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.