Mid-cap stocks -- companies with market capitalizations of between $2 billion and $10 billion -- can be a great source of investing outperformance. While smaller companies are valued for their outstanding growth potential and larger businesses are typically seen as less risky, mid-caps can offer the best of both worlds. That's because mid-caps are still small enough to grow at impressive rates, but large enough to be considered relatively low-risk investments.

If that sounds appealing, here are two mid-cap stocks that are particularly attractive investments right now.

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The fitness star

Peloton Interactive (NASDAQ:PTON) is helping thousands of people get fit in the comfort of their own homes. The maker of internet-connected exercise bikes and treadmills is expanding its customer base at a rapid clip -- and plenty more gains lie ahead for this $7 billion company.

Peloton added 149,000 connected fitness subscribers -- customers who purchased its exercise equipment and pay a monthly fee for unlimited workout classes delivered through its app -- in the second quarter alone. Peloton ended the quarter with 712,000 connected fitness subscribers, representing year-over-year growth of 96%. In turn, Peloton's revenue soared 77% to more than $466 million. 

Despite this impressive growth, Peloton's shares declined after its second-quarter report. Some investors were apparently concerned with Peloton's third-quarter guidance, which called for revenue to grow "only" 50%. But delivery timing issues and challenging year-over-year comparisons help to explain a significant portion of the revenue deceleration. Moreover, management raised its full-year subscriber and revenue growth guidance to approximately 81% and 68%, respectively, up from 74% and 61%. 

Some investors are also fretting over Peloton's soon-to-be-expiring lock-up period, which will allow early investors and insiders to sell their shares, should they choose to. But while expiring lock-ups does tend to result in volatility for recent IPOs like Peloton, any resulting sell-offs tend to prove short lived. If Peloton's shares were to pull back further, patient, long-term-minded investors may see it as a buying opportunity.

The real-estate disruptor

Redfin (NASDAQ:RDFN) helps home sellers save a lot of money. The discount brokerage charges commissions that can be as much as 66% lower than the typical seller agent fee. By saving its customers thousands of dollars in selling costs, Redfin is rapidly gaining share in the $82 billion U.S. real estate brokerage industry. 

Redfin's market share of U.S. existing home sales rose to 0.94% in the fourth quarter of 2019, up from 0.81% in 2018. That helped to fuel a 22% year-over-year jump in its brokerage revenue, to $496.5 million, in 2019.

Better yet, Redfin has several businesses that are growing even faster than its core brokerage segment. Its "other revenue" segment -- which includes sales from its title and mortgage operations -- soared 78% to $17.6 million last year. Its home-buying business, meanwhile, more than quintupled in size to $240.5 million.

Like Peloton, Redfin's $3 billion market capitalization understates its long-term market opportunity. Even after years of impressive growth, Redfin's share of the massive U.S. real estate market is only about 1%. But considering the enormous savings it provides to home sellers, there's no reason Redfin can't eventually capture a market share that's five or even 10 times larger. If it can, investors who buy Redfin's stock today could enjoy enormous gains in the years ahead.