I'm in the same camp as XGC, I have never been to D-Day, but have been to numerous Legends. There's a reason I plan all my vacation around attending Legends. The game itself is incredible. It's hundreds of people slugging it out on challenging terrain for all play types, whether you want to rain hell in the attrition war that is Bedlam/Armageddon, or if you want to sneak about and creep through the underbrush in the Ravine, there's something here for all playstyles. Year after year, all of the rules are looked at, tweaked and adjusted for a better overall experience. The CPX Staff, the Refs, and Viper always work their asses off to make it an amazing event, and the paintball game itself is worth going to, however Legends is so much more then that.
The Friday of Legends is where the party starts. People have started arriving as early as Thursday night (guilty of this myself last year), just to start hanging out and having a good time. Friday early on means you can get in and get your registration packet and paint tickets, and start checking out all of the amazing vendors and booths they have. One of my personal favorites is the Automags Online Old School Paintball Gun booth, where they always have something nifty to check out from paintball's past. They also have the huge Dye and Empire booths, a Planet Eclipse vendor shows up usually, plus Tippmann and HK Army too. I always bring extra cash for spending a bit and having some fun. Usually it's something silly and small, but its a nice little souvenir every year.
Friday afternoon/night is filled with the Open/Pump/Magfed games, as well as everybody's mini party, and my personal favorite (and something my team and I have helped grow in recent years), The International Beer Conference! Basically, people who love themselves beer and booze from all over the world that attend Legends, bring their local breweries beers, or homemade brews/booze concoctions, for swapping and trading and generally having a grand old time. This past year, we added a Beer Pong tournament, and this year we are looking to add Bags boards for public use for fun and having a good time!
Then Saturday Game begins, and the fight goes all the way through the day, Saturday night is the Bikini Contest sponsored by HK Army which is always a treat. Then Sunday game continues as sides are swapped, up until the FInal Battle! Final Battle is the most epic fight of any scenario game all season in my opinion. Afterwards is awards, and giveaways, and they give away some sweet guns and gear at Legends!
All in all, I think you'd have a great time at Legends, and if it's easier for you to get to, I highly recommend it. That being said, I myself plan to go to D-Day one year, just to say I did.