Dressrosa was a better arc, but Whole-Cake had a better climactic fight (sadly... Doflamingo got nerfed hard).
Dressrosa had a larger impact on the world, as the capture of Caesar put Big Mom into action in order to secure him, and the halt of Smile production put Kaido into action. Doflamingo revealed a lot of important plot points that are currently affecting the plot as we speak (906). We've been introduced to a new Admiral, and have another one mentioned. The Shichibukai system is put under question, alluding to a new system being implemented in order to replace them (later confirmed to involve Vegapunk developing something "incredible"). Imagine the affect this would have on Boa Hancock, Mihawk, and Buggy. The flare-flare fruit resurfaces, and Sabo returns to the plot. There's more development on the Blackbeard pirates. The Strawhats acquire a Grand Fleet consisting of 5000 members, considerably increasing their relevance and threat-level to the World Government (which takes place AFTER Whole-Cake). And there's a lot more.
What has Whole-Cake done? Give Nami and Sanji potential upgrades to their abilities? I mean, yes... the 3 Poneglyphs are going to important for developing on Luffy's goal, but what outside of this and him becoming a "5th Emperor" (lul) has affected the plot in a drastic way? Luffy's 1,500,000,000 bounty was not a result from merely defeating Katakuri and escaping from Big Mom's grasp, but from events taking place during DRESSROSA and before (Sabo connections, Defeating Doflamingo, Evading an Admiral, Grand Fleet, and previous achievements that the World Gov have been keeping secret, but are now slamming onto his threat-level)
I can guarantee you that Reverie Arc is going to be affected 100% by arcs taking place during and BEFORE Dressrosa. Whole-Cake itself has almost nothing to do with the plot as a whole outside of "Lel, Big Mom is ****** pissed". Except, she was already angered after Fish-Man Island.
Dressrosa Arc > Whole-Cake Arc