Here's my platinum chest drop frequency for each event.
Apocalypse (Expert spam): 1
Phoenix (Expert spam): 1
Garuda: (Expert spam, then Ultimate spam once it became available): 5

In addition, I noticed that Ultimate dropped a ton more of every type of material. Whereas I only had enough materials for 100 SP in the first and second events, I had enough for 300 SP from the Garuda event.

If you can clear Ultimate at least 2/3 of the time, I'd run that.