Well I was thinking more one instanced and the other not but you are on the same line. I do like the idea of PvP and PvE servers but not "Instanced" servers and non. Besides it wouldn't work if transfers were allowed which is pretty crucial to an MMO.
I was just talked into joining WoW with some coworkers as FFXIV continues to evolve. I can honestly say, the only thing I like about it at all is Battlegrounds and even those are getting boring. Granted I have only made it to 55 in the week that I have been on so I don't know how endgame raiding is but I hear stories from my friends and it sounds pretty scattered in contrast to FFXI's endgame. What I mean by this is that just about any group setup with a tank and a healer can down most bosses.
WoW caters to people too much. Warping from one thing to the next and playing with people from entirely different servers destroys any and all sense of community or cooperation. The only challenges in the game now are the instances themselves as you no longer have to navigate the world. It's really sad actually.
In the one week of playing WoW I am usually at the top of the honor chart in a battleground and I'm not used to hotbar style mmos. I failed at PvP in Aion a lot of the time and wasn't some crazy HNM person in XI. I say this only to point out how diluted WoW is and how little people work to learn their class. Everything is pretty much a smash and grab. It would be a travesty to see FFXIV evolve this way. Lucky for us it's not possible to fully learn you role because SE hasn't fully learned it either lol
Edit: In summary as long as the content is challenging and people have to learn their places, use a strategy. As long as instances are unique, rewarding and challenging I have no problem with the majority of content dropping in them. If I start seeing people running every instance with a "LFG Anything 1/4 or 1/8" you can count me out.