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.