I agree with VI. It's a matter of choice.
PlayDip's forum is far superior in terms of features. Although, for me personally, I like the simplicity of WebDip's.
PlayDip's messaging system is superior in some regards to WebDip's. As VI pointed out, your messages never get compiled into a long list crossed with messages from all powers as you talk more. It also lets you message multiple players at once, which is nice. Again, I prefer the instant messaging style here, but that's probably simply because I'm used to it.
The orders interface is terrible. I can't really defend it.
Again as VI pointed out, the ability to order impossible moves. It's not necessarily better or worse, but just different.
As for the paywall, personally I dislike it, but the merits are obvious. It allows them to actively pay to maintain their site rather than relying on potentially unstable donations.
At the end of the day, they both let us play Diplomacy. That's all that really matters. Anything else is just extra.
All that being said, WebDip is our home. I'd say that for the most part, that's what keeps us here rather than features.
Although I do have to say that it seems soloing is easier on PlayDip. I've played only a singular game over there and I ended it with 18.