The games certainly share a lot of mechanics, but both feel very differently. The 7 day turn was a big adjustment for me, though, since CW2 was my first Ageod game. Roughly, everything that applies for CW2 should apply, barring some exceptions I'm still learning. Defensive posture in the field seems less effective than for CW2, but most of my campaigns as Spain so far have been entirely offensive in nature.
So, my recommendation is to go for whatever era appeals to you most. I love playing as the Confederacy, for instance, because I really like Lee and the Army of Northern Virginia. I also like playing the loser and making them the winner, and Spain in the Napoleonic era allows that in spades. Honestly, the historical freedom of WON really makes it shine in my eyes, since I can launch invasions of neutral countries, form ahistorical alliances, and play in a veritable historical sandbox.
For instance, I'm contemplating an invasion of the United States. I have the manpower to seize the East Coast, but can I hold that much abroad with the British looking for reasons to attack me?
You ask different questions in the games. You think differently in them. Especially since there is no end to the war in CW2 except the end of the game. In WON, only Great Britain and France are hostile the entire time, unless by player choice.
And the diplomacy is not actually that complex- it's just another new window with a few options to click. Mastering it will doubtless take some time, but where it will really shine is PBEM, where players can conspire via email against one another. So, I wouldn't say that the way the game works out on individual turns is that much more complex, but it is more complex overall.
They are pretty different games, ultimately. So again, go for what you feel most passionate about right now, and if you need help there is a ton of people here to help.
Remember the golden rule- have fun!