Valuing the stocks of companies that sell trendy clothing can be a (small-f) fool's game when new sales depend on something as vague and fickle as the clothing tastes of teenagers. Companies targeting younger consumers, like Abercrombie & Fitch (ANF 0.95%) and American Eagle Outfitters (AEO 0.18%), are examples of the kind of company I am talking about, but there are many similar options, none of which look very attractive to (big-f) Foolish long-term investors. 

Source: Proud2bme.org.

Buying into these trendy-clothing stocks is inherently challenging because they are just that, trends, and often trends with little repeatable reason as to why one catches on, while another doesn't.

Additionally, these companies have seen a lot of pushback in recent years for the way their marketing and advertising marginalizes populations of teens and young adults who don't meet the image the companies try to project. The brand names themselves are even becoming less valuable, to the point that Abercrombie & Fitch is now producing more clothing that doesn't have its logo on the item at all, hoping instead to raise sales on generic, un-branded clothing.  

But investors don't have to abandon the field of popular clothing just because the teen-centric companies face challenges.

There is one company selling popular clothing -- the most expensive stock of the group I'm looking at -- that is worth buying. Under Armour (UAA 1.81%), at a P/E of 78, is not in the same "trendy" category as the teen retailers mentioned above, making this "athlesiure" clothing company much better positioned for steadier, long-term growth.

Fickle trends, fickle stock prices
During the early 2000s, there were multiple successful trendy clothing brands able to make vast earnings from the high profit margins the companies were awarded for managing to sell "hip" clothing at huge mark-ups, even though the clothing was produced in the same offshore factories as their value-priced competitors' offerings.

However, in the last few years, these companies have experienced a lot of variation in stock price because of fickle consumer tastes.

Look at the chart below, which shows how fickle the stocks of Abercrombie & Fitch and American Eagle Outfitters have been in the last 10 years, compared with the steady rise of UA. 

UA Chart

UA data by YCharts.

More, and better product segments
One way Under Armour is keeping itself safe from the fickleness hurting other apparel companies is by focusing on growth in multiple product segments, not just apparel. To be sure, the company's apparel segment has done well: It was up more than 26% year-over-year in 2014. 

Posting much higher growth than overall apparel, though, is Under Armour's footwear segment. Under Armour has released an array of shoe styles available for many different sports over the years. Most recently, the company's release of its SPEEDFORM line of running shoe in 2013, with an updated Apollo version released in 2014, has done very well for the company, helping to grow UA's total footwear segment revenue by 50% in 2014.

Source: Under Armour.

Another area Under Armour is going after is technology and consumer health. The company's recently released "Record" mobile app gives consumers a way to track exercise and health metrics, while also finding and competing with friends. This certainly isn't a new idea; UA is only now starting to catch up with Nike's (NKE -1.26%) platform. However, with a planned release of its own hardware device to go along with it, this is just one more way Under Armour is protecting itself from the threat of fickle trendy apparel tastes that its apparel-only counterparts have been hurt by. 

The most expensive, but the most inspiring
Under Armour posted net income up 22% year over year in 2014. While Under Armour is by far the most expensive of this group by the price-to-earnings multiple, that expensive stock comes with a much better chance of future growth than these other trendy-clothing stocks, based on the company's growing market share.

UA recently overtook Adidas to become the second-largest athletic apparel company in the U.S. by market share. And it's not just U.S. growth that looks promising, but international growth as well. International sales grew 90% year over year in 2014. Although those international sales account for only 9% of total net revenues now, Under Armour's management plans to eventually grow international sales to as much as half of UA's total revenue.

UA vs. Nike?
While Under Armour has seen incredible growth in the last few years, Nike is still by far the industry behemoth. And even though having somewhat saturated the market usually means a company has seen the last of its major growth periods, Nike has managed to post high growth in the last year as well -- as much as a 10% increase in year-over-year earnings in its fiscal 2014 ended May 31, 2014.

Another plus for Nike is that its share price is only 30 times earnings, compared to 78 times for Under Armour. However, with more than half of the market share, Nike is such a large force in the market already that, even though its income growth is impressive, it's not likely the company will ever be able to post as much growth as a company like Under Armour, which has been so successful with just a small share of the U.S. market, and an incredible opportunity for massive growth internationally. 

A Foolish final thought on value in the apparel industry 
"Trendy" clothing companies on the whole just don't look like good investments now. Of the large group of such companies, the ones that stand out most as the face of this industry, Abercrombie & Fitch and American Eagle Outfitters, just aren't appealing. Instead, the athletic wear space looks like a much better pick for popular-clothing companies going forward. 

Don't get me wrong -- Nike does look good right now. But don't discount Under Armour because of how much more expensive it is. Even though it is the most expensive stock selling popular clothing, Under Armour still looks like an opportunity for more big wins going forward.