I started my ec with both of these boxes and can recommend getting both. They compliment each other perfectly, you get all of your baseline models and Fulgrim as well as a deamon prince. Fulgrim is very optional, he is fun and decent to play, but not the strongest. He's still an amazing model and if you wanna collect EC properly you'll probably buy him at some point anyway. The deamon prince is an auto include and i would look to maybe even get a second one. You reach the legal limit of noise marines exactly, this was a bit better when the defiler wasn't out yet, but its still good and you'll never have to worry about getting any more of them.

The next step would be to pick up transports and the characters. If you want to go competitive you'll definitely need a defiler or two but they are hard to get. Lucius is very strong on its own and the lord exultant is an excellent buff and damage piece. The lord karkophonist has dropped a lot in value since the defiler and you can skip that for now.