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Why GM Shares Could Double

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USA! USA! At least, that's what car company investors seemed to be chanting on Tuesday.

Detroit's "big three" automakers saw their strongest March sales in five years, with U.S. sales at General Motors, Ford and Chrysler seeing year-over-year gains of 6.4 percent, 5.7 percent, and 5 percent, respectively.

Meanwhile, Toyota's U.S. sales rose just 1 percent, and while Honda's jumped by 7.1 percent, this is below the 8 percent it had forecast.

(Read More: Buckle Up: Auto Sales Surge in March)

Not surprisingly, shares of GM and Ford rose on the day (Chrysler is privately held), while Toyota and Honda dropped.

So are we seeing a great turnaround for U.S. automakers?

Morningstar analyst David Whiston certainly thinks so. "People like to write off Detroit as not making good cars," Whiston said, "but that's just not true anymore."

So where should you put your money?

While Whiston likes both Ford or GM, he thinks GM is the far better bet.

"Ford and GM are both in turnarounds, but Ford is further along," Whiston said. "I don't think people realize what GM can do once they get up to speed."

(Read More: Trader Bets GM Will Get Into Gear)

Whiston points out that "GM's key U.S. market is still growing, and they have a very robust practice in China." But while he can find plenty of reasons for GM investors to be optimistic, for this analyst, it's ultimately all about the cars. "GM's car portfolio is just night and day better than it was even a few years ago."

Whiston's "fair value" estimate on the stock is $52—a healthy 85 percent higher than where shares find themselves today.

Isn't that target a little aggressive? Not at all, he said. "If anything, I think that could be pretty conservative."

—By CNBC's Alex Rosenberg