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. Image source: Getty Images. 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. 10 stocks we like better than RedfinWhen investing geniuses David and Tom Gardner have a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* David and Tom just revealed what they believe are the ten best stocks for investors to buy right now... and Redfin wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of December 1, 2019 Joe Tenebruso has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Peloton Interactive and Redfin. The Motley Fool has a disclosure policy.Source