I had a somewhat backward experience compared to the OP.
I started in one of the Official PVE Servers. But problems with the population full at 40/40 all the time meant I was barely playing my character eventually.
So I elected to go to a different, Private PVE Server. Was doing fine, but now the Private Server is gone. Well sh*t.
So now I started a Single Player game.
Observations:
The people I ran into in the Official PVE Server were decent. People kept to themselves but on occasion, someone gets on chat to get people together and adventure. Since this game is new going Live, Dreggs was a frequent. People grouping up is quite important because you need the strength of multiple players, unless you’re some elite veteran, to take down these bosses. Some of the bosses are requirements in the “Journey” tab. Abysmal Remnant was the first one and I got rocked badly when I tried to solo it. Get some people together and it’s more feasible. There tended to be at least 1 guy in the group that knows what’s up.
Another benefit on an online server, regardless of PVE or PVP, is that even though you may not necessarily be adventuring with someone, I do enjoy seeing other players doing their thing out there. I know I am not the only one out there. It’s also cool to come across some of the creative housing, bases people make out there, which makes my shack look real bad.
There’s another subtle but pretty big thing with the Online vs Single Player gameplay, is crafting. The problem is if you’re doing a mass crafting effort an processing a lot of stuff. If it’s going to take forever, with an Online Server, you can log off for the day and come back, the crafting process is done and you got your gigantic stack of items waiting for you. But with a Single Player, if you log off, time stops and only resumes when you log back into the SP game. The crafting progress is exactly where you left off. Tweaking things like “Crafting Time Multiplier” left and right on the bar does nothing. I don’t want to keep the computer running all day with the game running just because I’m waiting for my stack of materials to finish processing.
Of course, the great benefit of Single Player is that you can customize things to your content. Tweak “Server” settings to what you want, implement whatever mods you want (or not, your choice).
But a warning again on Single Player, the Bosses were designed for more players to deal with them, so one player trying to do this requires some… finesse. A lot of it, actually. There is a mod that cuts down on the HP of bosses to make them more feasible in Single Player. It hasn’t touched the regular NPCs.