Billionaire businessman and publicity hound Donald Trump blew it last night. In the finale of season four of The Apprentice, his choice came down to two highly capable and motivated candidates: Chicago-based writer Rebecca Jarvis, and New Jersey native and management consultant Randal Pinkett. In the end, Trump chose Randal. And it was a huge mistake.

That's not to say Randal shouldn't have been hired. He's a Rhodes scholar, a graduate of the Massachusetts Institute of Technology, and a thriving entrepreneur. And his 3-0 record as project manager was indeed impressive. But so was Rebecca. She endured the final 12 weeks of the interview while on crutches. She consistently showed guts and intelligence. And, except for kowtowing to Yahoo! (NASDAQ:YHOO) executives, she was brilliant in her final task. I'd almost argue that she won the head-to-head competition, except that her team didn't raise any money for the Elizabeth Glaser Pediatric AIDS Foundation. (Though, in the end, Yahoo! donated $50,000 to the charity, which is probably more than the event -- had it been a real fundraiser -- would have drawn.)

Which makes Trump's choice all the more baffling. I mean, really, didn't you think that in a season filled with mass firings, The Donald might be ready to hire them both? Even Trump majordomo George Ross said that it was a choice between two stars, and you could tell Trump agreed. He even asked Randal if he should also hire Rebecca. Trump's new employee, however, said no, arguing there could only be one apprentice.

Amazingly, The Donald bought it. What? Kelly Perdew isn't a Trump apprentice? How about Kendra Todd? Anyone else remember Bill Rancic? Trump had two big jobs available. Randal chose to help reinvigorate the ailing casinos run by Trump Entertainment (NASDAQ:TRMP). Rebecca wanted to run a New Jersey high-rise condominium development. There would have been no inherent conflict had both been hired. But Trump wilted, and Randal got his wish.

The audience, however, hated the decision. Booing was audible as Randal turned to acknowledge them. I agree, and not just because Randal backstabbed a colleague he said he respected. More importantly, I believe, Trump turned away serious talent that wanted badly to work for him. Sadly, though, such decisions are hardly uncommon. Investors do the same thing every single day.

Allow me to explain. When given a choice between several comparable and undervalued stocks, investors will inevitably opt for the one they believe is the best, even if the evidence suggests all of them could be major winners. It's easy to understand why. In our newsletters, our analysts confine themselves to no more than two picks each month.

That is not, however, how the best money managers treat their portfolios. They understand that it's fine to own or buy both Apple (NASDAQ:AAPL) and Dell (NASDAQ:DELL), or any other host of good opportunities at a single point in time -- not just a choice pick or two.

That's where Trump went wrong last night. His either/or choice was arbitrary and unnecessary, and he hurt his organization as a result. Don't let that same mistake plague your portfolio.

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Fool contributor Tim Beyers still thinks The Donald has the best combover in the business. Tim didn't own shares in any of the companies mentioned in this story at the time of publication. Dell is a Motley Fool Stock Advisor pick. You can find out what's in his portfolio by checking Tim's Fool profile. The Motley Fool has an ironclad disclosure policy.