The Amazon Echo, left, and Google Home Source: (L to R) Amazon; Google

When it comes to voice assistants, Amazon Alexa and Google Assistant lead the pack by far. With reports that Alexa may be losing dominance, the competition between the two is only going to get more intense. CNBC spoke to five ad agencies and who have worked on projects on both devices on behalf of marketers, usually creating branded apps for the voice experience. Here's the pros and cons they've seen for each device.

The case for Alexa

Alexa everywhere. Alexa has entered far more homes and offices. About two-thirds of U.S. smart speaker enthusiasts will use an Amazon Echo according to eMarketer, with Google Home coming in at just 30 percent. But Amazon wants Alexa to be in as many devices as possible rather than just its own products, according to Chris Neff, senior director of innovation at advertising agency The Community. Even if Alexa-enabled devices created by Amazon are losing market share, it could continue to gain ground through Alexa partners, he pointed out. “The biggest strength of Alexa, of Amazon, is right now the market penetration and their goal of getting Alexa into everything and everywhere,” Neff said. A seamless retail experience. Alexa is tightly integrated into Amazon's core shopping platform. “The most expensive ad space in the future will be Alexa,” digital agency Huge global executive creative director for innovation Sophie Kleber said. “They are really just integrated in the shopping platform.” This has also opened up the door for marketers to sell items through Alexa apps. VaynerMedia worked on converting popular mobile game “Heads Up!” for Alexa, and was the first to integrate a voice-activated one time payment functionality to buy add-ons. “That’s a whole different way to monetize work,” said Patrick Givens, vice president and head of IoT and emerging tech division VaynerSmart. A developer ecosystem. A couple agencies also pointed out Amazon has been very open to experimentation, and has invested quite a bit in tools for developers like Amazon Voice Services. In addition to testing out payments, apps like Gimlet Media’s Chompers – a two-minute podcast to get kids to brush their teeth – has a natural home on Alexa for Kids. An interactive audio story made by Earplay and Universal for “Jurassic World: Fallen Kingdom” lets players dodge dinosaurs using the Amazon Echo's buttons. “More developers are coming into the Alexa ecosystem, because they’ve made it easier for developers to create skills and apps,” said Dan Calladine, media and marketing agency Dentsu Aegis’ head of media futures. “What we are starting to see is more creativity in the space.”

The case for Google Assistant

Better language processing. For Alexa and Assistant to become real voice assistants, Neff said they'll need to be able to nuances of how people speak, where they are located, and what they would like based on their past history. For example, it should be able to understand Neff is living in Miami, and based on what he frequents, know not only that he likes Hispanic food but exactly what type of cuisine. "Now that I'm able to ask these smart devices recommendations, I would want to ask them to tell me something interesting to me," Neff elaborated. "We're delving more into it being an assistant and less of it being a transactional device." While Alexa has an advantage in e-commerce, agencies say Google Assistant is better at natural language processing thanks to Google’s history of being a search engine. It has a leg up at understanding how people speak and finding them the right answer.