I've only played Worms World Party of all the worms and have to say that it's a blast. I can't imagine how a previous version of the game could be any better. What major differences between the two, if any, are there? Are there more people online on Armageddon than on World Party?

http://www.nanacide.com/wahelp/info-better.php

So is there a large amount of people who still play the game?

A few hundred, surely. Not all of them visible in the community agglomeration points, but they're there. Around 20-30 of them are usually to be seen in the ingame channels, some more currently having a game. So yeah, the game is not as active as Counter-Strike, but one can't complain. ;)

w:a is currently the newest 2d worms game for the PC thanks to several updates.
wwp is the most recently published 2d worms game for the pc.

changed to suit etho.

Total Misinformation.

Worms XBLA (http://www.xbox.com/en-US/games/w/wormsxboxlivearcade/default.htm) is actually the most recent 2D worms game. Worms World Party is the most recent PC 2D worms game. Worms Armageddon is the most recently updated PC 2D worms game.

Worms Armageddon introduces many excellent new features if you patch it correctly. I'm sure you will enjoy it more than Worms World Party if you do get it.

what's the latest patch? I keep getting an error saying my other copy is not the same version, even though it's 3.50b2

www.nanacide.com/wahelp/ has all the info. And a shortlist of added goodies [which you've already been linked to by Wyv].

But you can only apply it to CD versions of the game. Trymedia = ripoff artists.

what's the latest patch? I keep getting an error saying my other copy is not the same version, even though it's 3.50b2
Didn't you get the Trygames version?
Then this is the latest version for you, as it cannot use the beta updates.

The latest public version is 3.6.26.5. Download the update here (http://www.nanacide.com/wahelp/WA_update-3.6.26.5_Beta_Installer.exe).