Less than six years ago, Citigroup (NYSE:C) took a $45 billion bailout as the government worked to prop up the unstable financial giant. In stark contrast, today's Citigroup is in turnaround mode as it's once again reporting billions in annual earnings. But here are ways beyond its common stock to invest in Citigroup, which is right for you? Well, it really depends on what type of investor you are.

Value investors
These investors are naturally attracted to Citigroup's common stock. Trading below book value, this bank already checks off one box on the value investor checklist. Making the value even better, Citigroup is also the only one of the four major U.S. banks to trade below tangible book value.

Normally, a company with these valuations would have to be losing money or have serious solvency concerns, but Citigroup has neither. Earnings are positive and continue to grow -- in fact, the bank trades at a single-digit forward price to earnings ratio. Citigroup is also adequately capitalized reporting a 10.5% Basel III Tier 1 capital ratio for the end of 2013 -- a level on par with more highly valued peers.

With Citigroup common stock trading at its current multiples, value investors don't have to look far for their ideal Citigroup investment.

Unlike value investors, speculative investors are looking for bigger returns, even if they mean more risk. Fortunately, Citigroup offers a wide array of ways to bet on its success.

Aside from the common stock, Citigroup also has two classes of publicly traded warrants. Issued during the TARP bailouts, these warrants were originally given to the government to allow taxpayers to see some of the upside in Citigroup common stock. But as the government moved to exit its Citigroup position, both classes of warrants were listed on to the NYSE.

Both classes are well out-of-the-money today, but offer impressive amounts of leverage for long-term speculators. Citigroup Class A Warrants (NYSE: C-AW) have a strike price of $106.10 and expire Jan. 4, 2019 and Citigroup Class B Warrants (NYSE: C-BW) have a strike price of $178.50 and expire Oct. 28, 2018. Investors should also bear in mind these warrants are for 1/10 of a share each due to Citigroup's 1-for-10 reverse stock split.

Both warrants require Citigroup to more than double from today's price. If banks can return to historical valuations, the Class A warrants have a decent chance of being in-the-money by expiration. The Class B warrants are even further out-of-the-money, making them extremely speculative and have a large chance of expiring worthless.

Income investors
Bank stocks have traditionally been income investments, but the past several years have dealt heavy blows to bank dividends. With Citigroup common stock yielding less than a tenth of a percent annually, income investors should take a look at the preferred stock instead.

Citigroup, like many other large banks, has a large number of preferred stock series to pick from. One example carrying a good yield and some potential upside is Citigroup Series C Preferred Stock (NYSE: C-C). Trading at $21.58, this preferred yields 6.7% providing some upside in the event of a call but also a yield greater than most other income investments in big banks.

Investors should note that this series, and many others from Citigroup, are non-cumulative meaning that if the bank misses a payment, it's not obligated to make it up. However, this would really be a last resort action for Citigroup as it would require eliminating the dividend on the common stock while simultaneously raising a big red flag for investors. Citigroup can also call Series C but can only do so for $25 on or after April 22, 2018. With current discount to the $25 liquidation value, a call would mean additional gains from today's price.

Investing in Citigroup
Citigroup got hit hard during the meltdown, but has fought most of the way back. Value, speculative, and income investors can all find a way to invest in this bank by choosing the right security.

With shares trading below tangible book value, a rare opportunity in publicly traded warrants, and yields higher than most rivals, making an investment in Citigroup could be a great long-term investment.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.