At The Motley Fool, we poke plenty of fun at Wall Street analysts and their endless cycle of upgrades, downgrades, and "initiating coverage at neutral." Today, we'll show you whether those bigwigs actually know what they're talking about. To help, we've enlisted Motley Fool CAPS to track the long-term performance of Wall Street's best and worst.

Focus on 3-D
For investors in the burgeoning industry of three-dimensional "printing," it's the eternal question: Should you buy shares of eponymous stock 3D Systems (NYSE:DDD), or of upstart rival Stratasys (NASDAQ:SSYS)? On the one hand, 3D just introduced a new product -- a "VIDAR DiagnosticPRO Edge" scanner that supposedly "doubles scanning speed for enhanced productivity" in 3-D manufacturing. On the other hand, though, Stratasys hasn't exactly been sitting on its hands, either. The company just finished reporting a blockbuster earnings quarter, and upped its earnings guidance for the rest of the year. Which to choose... which to choose?

Yesterday, investment banker Piper Jaffray shrugged and made a modest suggestion: "Why not just buy them both?"

When in doubt, punt
Upgrading both 3D and Stratasys to "outperform," the analyst slapped a $76 price target on Stratasys, and imprinted a $55 target to 3D. Problem is, no one seems to know why. While most mainstream news outlets agree that the upgrades happened, I've yet to see any details on precisely why Piper thinks these two stocks will outperform the market.

That's a crucial question because, as even the stocks' fans must acknowledge, shares of both these stocks look pretty pricey. Buy a piece of 3D, for example, and it will cost you a steep 62 times trailing earnings. Meanwhile, a share of Stratasys will set you back a good 77 times earnings.

I mean, sure, both stocks are growing quickly. Analysts predict 3D will see 14% annual earnings growth over the next five years. Meanwhile, Stratasys' alliance with Hewlett-Packard (NYSE:HPQ) appears to be helping the smaller company outgrow its rival; analysts put Stratasys' growth rate at 15%. But, while certainly respectable, neither of these growth rates comes anywhere near justifying the nosebleed P/E ratios we're seeing. But could Piper be seeing something the rest of us aren't?

Paying Piper its due
I suppose it's possible. After all, ranked in the top 20% of investors we track on CAPS, Piper Jaffray is arguably one of the better analysts out there today. Problem is, that may not be saying much. On average, most analysts actually underperform the S&P 500 on their stock recommendations. And as it turns out, Piper -- although better than average -- also gets more picks wrong than right.

According to our CAPS records, only about 47% of this banker's stock recommendations actually succeed in exceeding the performance of a plain vanilla S&P 500 index fund. Worse, when it comes to picking "Machinery" stocks, such as 3D and Stratasys, Piper's record of "success" actually declines to just 36% accuracy.

Which you have to admit, bodes poorly for the prospects of its new 3D and Stratasys picks.

Foolish takeaway
Numbers don't lie, Fools. Piper's record of poor performance in the machinery industry is a bright red flag, waving you away from pretty much any stock the analyst says it likes. Not that you needed the warning, in this case. When a stock's selling for upwards of 60 times earnings, but showing a growth rate barely into the double digits, that's a warning siren, too, and it sounds loud and clear: "Don't buy."

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.