Ugh...this is tough.
For Enies Lobby, you get great focus fights for ALL the Strawhats, everyone gets to do something during that arc. The fight between Lucci and Luffy is probably still one of the best fights in the series. You have Robin's declaration of wanting to live. You have the Merry showing up to save everyone before having to say goodbye to everyone. Enies Lobby is one of the most emotionally impactful arcs in the entire story.
For Marineford, you have ALL the biggest names that in this world showing up for a big fight. We finally get to see some of these characters that have only be hinted at beforehand. And they all actually do stuff. We have Whitebeard forgiving his traitorous son. We have the declaration of the One Piece being REAL! We get Blackbeard getting his second Devil Fruit. Admirals, Shanks, Mihawk! We get the first real death in the entire story, completely changing the tone and stakes of the story going forward. It's just a really big event that ends the first half of the story in a big, bombastic way.
I think I have to give it to Enies Lobby in the end. If only because I think it's a better representation of the series as a whole. Enies Lobby gives us great moments with ALL our main characters. Marineford is great, but Luffy's really the only main character who shows up in that arc. All the other Strawhats get sidelined while Luffy alone gets the focus. Marineford is great because it subverts the usual One Piece formula. But, Enies Lobby is great arguably because it's the purest form of the One Piece formula.
They're both pretty great. But, I have to give the edge to Enies Lobby.