College diploma and cash © Corbis

One of the most popular college savings vehicles -- with good reason -- is the 529 plan.

Offering federal and, in some cases, state tax advantages, 529 plans are low-maintenance, provide tax-deferred growth and make less of an impact on a student's financial aid package than assets stored in checking and savings accounts.

These plans are also complex, and choosing the one that fits your family can be difficult. Know these seven things before opening a 529.

Some are pricier than others

According to the SEC, 529 plans come in two flavors. First, there are prepaid plans that allow families to buy "units" of tuition at a rate close to today's prices. They are cashed in when the student attends school. Second, there are 529 college savings plans that allow families to invest in preselected investment portfolios that grow (or shrink) in accordance with the markets. Many states and the District of Columbia offer 529 college savings plans, according to Morningstar. But only a handful of states offer a prepaid option.

Both types of plans come with annual fees. For college savings accounts, these range from about 0.24% to 1.88% of invested assets, according to a 2011 Morningstar survey.

When comparison shopping 529s, Richard Norman, the former interim executive director of the Ohio Tuition Trust Authority, which oversees that state's 529 plan, advises families to be aware of the price difference between direct-sold plans that families manage themselves and broker-sold plans that are overseen by a financial professional.

"In almost every single instance, (direct plans) will be cheaper and will give you more return than that through a broker," says Norman.

Broker-sold investment options cost 1.47% on average versus 0.54% for those investors can buy themselves, according to Morningstar.

There's risk involved

Many 529 college savings plans contain mutual funds that are tied to the stock and bond markets, meaning families can potentially lose money if their investments take a hit.

"The question you have to ask yourself is: 'How diversified is the 529 itself?'" says Sam Mikhail, the president of College Planning Advisors, a college planning firm in Burbank, Calif.

When deciding on the right college savings plan, Mikhail advises families to first consult with a financial professional on whether a 529 plan is the best bet, then examine the underlying funds in a particular plan and investigate each fund's historical performance. There are generally several investment options for you to choose from in 529 plans.

Families may also want to check out the growing number of no-risk 529 plan options. Several states, including Virginia, Arizona and Ohio, have FDIC-insured CDs and savings options that provide families with 529 tax benefits without the risk (or growth) associated with market fluctuations.

They might not keep up with your kid

Among the most popular 529 college savings options are age-based target-date funds that automatically shift from aggressive to conservative investments as the child approaches college age. Stuart Ritter, a vice president and certified financial planner with T. Rowe Price Investment Services, advises families eyeing age-based portfolios to examine how they change over time.

"It's important to understand what the investments are, and make sure that how your money is allocated between stocks, bonds and short-term investments is appropriate for the time horizon you're looking at," Ritter says.

How fast investments shift varies among 529 plans. To ensure your age-based portfolio is keeping up with your savings needs, Ritter advises families to evaluate several age-based portfolios before choosing one and to periodically re-evaluate how their 529 plan is evolving. If a plan isn't changing according to your needs, the Internal Revenue Service allows families to change portfolios once per year.

Your state's plan might not offer the best deal

While all 529 plans come with federal tax advantages, certain states also offer state income tax deductions or credits, too. These can be substantial.

Indiana, for example, offers a tax credit of up to $1,000 per year. Meanwhile, New York and Oklahoma offer annual tax deductions of up to $5,000 and $10,000, respectively, for single filers.

A few states, such as North Dakota and Rhode Island, sweeten the deal even more by offering matching grant programs that match a certain amount of a family's contributions. Eligibility requirements for tax advantages and matching grants vary by state.

While these incentives can provide an extra push to invest close to home, families should still compare different plans and evaluate whether tax incentives are enough to make a home state's plan the best financial bet, says Deborah Goodkin, a managing director of college savings plans at First National Bank of Omaha, the entity that oversees Nebraska's 529s.

"If (a 529 plan offers) tax advantages, it may or may not be the best reason to invest in your home state's plan," she says. "That's up to the investor and their financial adviser."

Independent investors may want to take a close look at fees, which can make a big dent on investment returns.