
As a guild, we have been using Teamspeak for a number of years now. We managed to get a special licence to have a free, very large voice server which is kindly hosted and maintained by one of our members. This requires some work on his behalf as he needs to keep ensuring it is up to date, working and has the licence renewed regularly.
Having this server has many plus sides .. including the size; the fact it's free; it's stable and it is "in-house" ... we're not relying on some large corporation in the US, for example, to keep things running - if there's a problem, we can contact the man in charge direct 
It does also seem to have a few problems though - less and less people appear to be using it "as default", so new members have to download new software, and then set it up - which isn't always easy or obvious; the overlays seem to be causing problems - some reporting lag spikes and framerate drops when using it, some struggling to get the overlay to work at all.
We are seeing more and more people use Discord, and indeed we do have voice chat channels set up that we used once when there was a Teamspeak problem, and that are used fairly regularly for dungeons and raids that are not our standard Wednesday/Friday raids.
So I'm considering switching over to using Discord as our "default" voice chat and, for the time being, keeping Teamspeak as a backup.
Having said that .. I don't use Discord voice chat regularly, so would like opinions for and against both options. I don't know how stable Discord is, what the sound quality is like on a regular basis and how well the overlay works, compared to Teamspeak.
I've added a question regarding this to the guild raids channel in our Discord, but am aware that not everyone is on our Discord so would also like comments here.