I'm not terribly familiar with Google Voice (it also isn't available in my country...), but they look similar in terms of functionality at this point in time. For me personally, the primary reasons to go with Relay would be that I'm already trying to move away from Google as much as possible for privacy reasons, that I'm already using Relay for email masking, and that Relay is explicitly focused on the privacy use case and will keep evolving in that direction.

I can relate to the privacy-focused goal of getting away from google however Google Voice is free. Sadly I think having a competing free Google product that accomplishes most of the same things is going to hurt adoption of the Firefox relay product (which is paid)