Many investors would rather keep their money in their pockets right now. Equity markets have been volatile, the near term looks somewhat uncertain, and President Donald Trump's international trade policies might lead to more severe economic problems down the line.

However, it might be worth investing in stocks, even in this situation, if you can afford to do so. If you have $5,000 that isn't being saved for routine bills or a rainy day, here are two solid companies whose shares are worth holding on to: Intuitive Surgical (ISRG -0.13%) and Booking Holdings (BKNG -0.49%).

1. Intuitive Surgical

Intuitive Surgical leads the market for robotic-assisted surgery (RAS) systems thanks to its famous da Vinci system. This machine aids in performing minimally invasive surgeries -- it uses tiny instruments to make small incisions in patients' bodies to access internal organs. That way, surgeons can avoid cutting open skin tissue, as is usually the case for open surgeries. The da Vinci system is cleared for a wide range of procedures, including urologic, colorectal, cardiac, bariatric, and many more.

Intuitive's revenue and earnings have grown substantially over the past decade, coinciding with an increase in procedure volume:

ISRG Revenue (Annual) data by YCharts.

The long-term view for the healthcare giant is attractive for several reasons. First, there's an aging population and an underpenetrated RAS market. That will lead to an increased need for surgeries, including the kinds of minimally invasive ones the da Vinci system can help with: They lead to less cutting of the skin, less bleeding, less scarring, and faster recovery time.

Second, even though competition in this market will become fiercer, Intuitive has a significant advantage. Its da Vinci systems cost between $700,000 and $2.5 million, a substantial investment for healthcare facilities. After spending all that money and a considerable amount of time to train their personnel on Intuitive's equipment, they won't want to switch to a competitor's. In other words, Intuitive Surgical benefits from switching costs -- a powerful moat.

Third, Intuitive should continue to innovate. The latest generation of its da Vinci system (the fifth), which earned clearance in the U.S. last year, is an improvement over previous versions. The company is also increasingly implementing artificial intelligence across its business.

Intuitive Surgical has crushed the market in the past, but good days are still ahead. At current levels, you can purchase nine of the company's shares for $5,000; holding on to these for the long term should lead to outsize returns.

2. Booking Holdings

You could only buy one share of Booking Holdings with $5,000, but it would be money well spent. The company is a leader in the hospitality industry. So long as people love to travel, which won't stop anytime soon, there will be a need for the kinds of services it offers. Booking Holdings helps with travel accommodations -- including flights, hotels, car rentals, and activities -- via several websites.

Having a moat is arguably a necessary condition for a company to perform well over the long run, and Booking Holdings has one thanks to its platform's network effect. The more travelers use its services to find what they need, the more attractive it is for hotels, car rental companies, and other businesses to join in -- and vice versa.

Booking Holdings also arguably benefits from a strong brand name. Some of its websites (which include Kayak, Priceline, Agoda, and the namesake Booking.com) are well-known, and often among the first options people turn to when searching for various travel accommodations. That's why, despite increased competition, the company has continued to perform well:

BKNG Revenue (Annual) data by YCharts

Even though it faces some uncertainty due to the current economic situation, its long-term prospects remain intact. True, there will be less travel if a recession hits -- or perhaps even if people think one is coming. That would almost certainly harm Booking Holdings' business in the short run.

The company dealt with precisely this issue during the early days of the pandemic. But then travel recovered and its revenue and earnings got back on track, as did its stock price.

You can expect the same in the future. Booking Holdings' shares are worth holding on to even when the economy tanks, because it's likely to rebound and deliver superior returns over the long run.