In the end, they’re not all that different.
Discord is better from a casual community building perspective, with your members able to message eachother throughout the day and thusly develop relations be it through casual conversation, sending memes to eachother etc.
Teamspeak is more specialized in voice communication and organization, and is the way to go yet for more competitively oriented/“serious” regiments what with its higher quality voice comms and the ability to organize rooms/players more effectively.
Both have voice comms and both have the ability to chat with players via text, only one is more specialized than the other in one area while the other is better in the second.
Of course, you could also do a hybrid system which I see increasingly often- Teamspeak for events/voice comms, Discord for community organization and development. This will mean a bit more moderation having to monitor 2 places but it will also eliminate the need for steam groups and messaging there (for the most part)
So, just ask yourself what you’re looking to get out of this and the worth of what you’re going to get out of it. I personally would recommend a hybrid system, but only you can make the ultimate choice for your reg once it comes down to it.