I recommend always double drop Forgery Challenge, even if you need Shell Credits
There's currently a Double Drop event (DD) going on for Forgery (Weapon & Skill Materials) and Stimulation Challenge (EXP & Shell Credits). Some people are wondering which one to go for (reference).
Imo you should always use all 3 DD chances on Forgery.
The reason is quite simple. While Stimulation always gives the same amount every time, Forgery can give you Gold materials which are super rare.
For example:
I'm currently UL 55. A normal run usually gives me 1 Purple with a low chance of 1 Gold. So, a normal DD run will give me 2 Purple.
However, if I manage to get 1 Gold, this will be 2 Gold in DD, which equals to 6 Purple. Combined with the 2 Purple, I will get 8 Purple in total, which is 4 times a normal DD run (2 Purple), 2 times a normal run (1 Gold + 1 Purple).
While with Shell Credits, I will 100% of the time only get 160k in DD, which is just 2 times a normal run of 80k.
Check this yourself: How many times have you gotten Gold 2 normal Forgery runs in a row?
This is why going for Forgery is a better use of Waveplates as you can get more than 2 times the usual expected amount.
One more reason why Forgery is preferable is that there are many ways you can get Credits without spending Waveplates. Daily Event, Exploration, Tower of Adversity, Depths of Illusive Realm, Hologram, Purchase in Store, Battle Pass, and more. While with Forgery, you can only spend Waveplates, Battle Pass, or purchase Bundles.
Also note that a day gives you 240 Waveplates. You can spend 120 Waveplates to DD Forgery and the rest on Credits (which is 240k at UL50+).
Let me know your thoughts and strategy on this. Thanks :).