Twilio (NYSE:TWLO) and Line (NYSE:LN), two major tech IPOs of 2016, both had rocky market debuts. Twilio went public at $15 last June, surged to the mid-$60s three months later, then stumbled back to the mid-$20s. Line went public at $32.84 in late July, rallied above $48 three months later, dropped back below its IPO price, then eventually rebounded to the low $40s.

Line fared better this year, with its 27% gain easily outpacing Twilio's 12% decline. But is either stock worth owning in today's frothy market? Let's take a closer look at their core businesses, growth rates, and valuations to decide.

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What do Twilio and Line do?

Twilio provides a cloud communications platform for app developers. Its APIs (application programming interfaces) are used to integrate text messages, calls, videos, and other features directly into apps.

The company's customer base includes major tech companies like Facebook (NASDAQ:FB), Amazon, Uber, and Airbnb. Rising demand for its services has been fueled by the rise of "no stack" start-ups, which focus on an app's central feature (like Uber's ridesharing service) while outsourcing other features -- like integrated calls, texts, mobile payments, and mapping services -- to other companies.

Line's main product is a mobile messaging app that mostly serves Japan, Taiwan, Thailand, and Indonesia. The company generates its revenue from ads, paid stickers, themes, a Skype-like VoIP service, games, comics, songs, and other content.

Like its Chinese counterpart Tencent's WeChat, Line is evolving into a monolithic "super app" that enables users to access various services (like mobile payments, a camera, and a mini social network) without ever leaving the app.

How fast is Twilio growing?

Revenue rose 41% annually to $100.5 million last quarter, but that represents its slowest growth rate since its IPO. Analysts expect its revenue to rise 40% this year, marking a slowdown from 66% growth in 2016.

However, its core growth figures are still healthy. Its base revenue -- which it collects from customers who have entered 12-month minimum revenue contribution contracts with Twilio -- rose 43% annually to $92 million last quarter. This means that the company is gradually becoming less dependent on big "non-base" customers like Facebook.

Twilio's dollar-based net expansion rate, or the amount of revenue it generates per existing customer, also surged 122% -- indicating that it's successfully cross-selling new features to its customers. Its active customer accounts also grew 35% to 46,489.

However, Twilio still isn't profitable by GAAP or non-GAAP measures, and it hasn't offered a road map toward profitability. This is troubling, because new rivals like Bandwidth and Vonage's Nexmo are gaining ground, while big customers like Uber are developing in-house alternatives to Twilio's platform.

How fast is Line growing?

Line's revenue rose 18% annually to 42.5 billion yen ($380 million) last quarter, an acceleration from its 17% growth in the previous quarter. Analysts expect Line's revenue to jump 25% this year, compared to 17% growth in 2016.

Line's core growth figures seem solid. Its monthly active users across its four key markets (Japan, Taiwan, Thailand, and Indonesia) rose 4% annually to 168 million last quarter -- but that still marked a sequential decline of 0.6%, due to a combined loss of 2 million users in its three non-Japanese markets.

Line's ad revenue climbed 41% annually, its communication revenue grew 4%, and its "other" revenue (from mobile payments, job searches, wireless services, and other smaller businesses) jumped 38%. Those gains offset an 8% drop in its content revenue.

Unlike Twilio, Line is profitable. However, its net income fell 42% annually last quarter, due to higher operating expenses related to new products and services like its Wave smart speaker and its virtual assistant Clova. However, analysts still expect its earnings to grow 85% for the full year, although Line doesn't provide any official forward guidance.

The valuations and the verdict

Neither Twilio nor Line can be considered cheap stocks. Twilio doesn't have a price-to-earnings ratio, and its price-to-sales ratio of 6 merely matches the industry average for software companies. Line trades at a whopping 86 times trailing earnings and 52 times forward earnings.

I personally wouldn't buy either stock at these levels, but Line's accelerating sales growth, expanding ecosystem, and consistent profitability makes it a safer play than Twilio. However, Line could still struggle to counter bigger messaging players like Facebook over the long term.

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.