I am by no means an expert user of either Nextcloud or Pydio, but I have looked at both of them so here are some thoughts.
Nextcloud has an active and very helpful forum community. Much more welcome than ownCloud was (just my opinion) and I like some of the perks like customizing it for our own logo. I’ve been using ownCloud for a couple of years now but I am migrating our office to using Nextcloud.
Pydio is also very active and helpful. Charles seems to be the main driving force behind Pydio and is very active with help and suggestions. He seems like a great guy! But having one person driving Pydio seems risky to me. I know that Pydio has been around for quite awhile (started out as Ajaxplorer) and I’m not suggesting it’s not going to last but still a point to make.
There was one instance though where Pydio solved a problem for me that Nextcloud (and ownCloud) couldn’t. In our small office we had a requirement to serve up PDF documents via a local webpage. This seems simple enough but has proved to be very difficult. I have all the PDF documents we wanted to be served up in folders on our NAS. Through Nextcloud I can setup a samba share to our NAS and ‘serve’ up the folder with all the documents in it. Worked amazing! The secretary could update the documents on the NAS and our local webpage shows the updates immediately! But we only wanted our employees to be able to read the PDF documents and not download them. Removing the download buttons on Nextcloud (and ownCloud) was next to impossible short of me digging into code. But Pydio made this very easy for us in that we changed from using PdfJs to imagick and within seconds we had a nice internal webpage for our employees to read all their standard operating procedures from.
But if you really want to see the difference between Pydio and Nextcloud then give them both a test drive. Want an easy way to test drive each software? I use DietPi on my Raspberry Pi. Within 20 minutes I can have a new O/S installed with both Pydio and Nextcloud running on my Pi to play around with.
They even have a virtual machine of dietpi to install and test it on.
Check out the list of Apps they offer to install through their wizard here: http://dietpi.com/phpbb/viewtopic.php?f=8&t=5#p5
I have no affiliation with dietpi…just found it recently and thought it was useful!
Good luck in your quest to find the answers you need on Pydio.