Hi Jeff,
Campaign Corinth was the first game in the series and so up to the 1.01 patch you could bring in other maps and OOB's, after that, and for all future games you can not. So, if modding is what you want to do then Corinth is the title you want. However, with that said, you can not apply the more recent patches and get the new game engine enhancements.
With each subsequent title scenario design is done with the existing maps & oob's. You can use the sub-map feature and crop out a segment of an existing larger map and effectively create a new map for your custom scenario. I use that feature a lot and it works quite well...check out the scenario I did for Round 2 of the current tournament...Round 3 also, but that won't be posted until that stage get's underway. Those can be found in the downloads section -
http://www.warfarehq.com/archives/forumdisplay.php?f=45
For sheer size, Campaign Gettysburg has the most to offer...a half dozen HUGE maps and it covers the most famous campaign of the war. Tons of scenarios, both stand alone, and for the campaign (variety) as well.
All of the games are interesting though, and it depends a lot upon your specific interest. There's enough content out there now with the 6 titles that you could play for years without ever having to create your own scenarios...but if your like me, I just have to create some of my own.
And finally, there is discount pricing available...contact me for details on that if you are interested.
Regards,
Rich