You should always use both or at least to a point.
You will always need to use Wu until you get Tarlak or some future hero of the same.
Wilbur is a different story. If you are using him for every Titan, it’s for survivability. He is a must if you are hitting titans that kill you in one or two shots. He also doubles as a defense down (another critical component to a Titan team) so you can trade out that Grimm for a hero with more tile damage.
As you get stronger hero’s, he can be phased out to obtain higher scores (depending on your skill level).
I’m still using both for all titans. We hit 10s and 11s and I typically always get A or A+.
Wilbur gives great -defense and +defense, and spreads incoming daamage among everyone to prevent one-shots.
The -def +att combo is the core of a successful Titan attack team, so with Wu and Wilbur, you get this pairing, plus Wilbur’s extras.
At 12* titans I use Tarlak instead of Wu except against purple titans. The big heal plus no-miss +att makes him compelling. I also use Athena instead of Wilbur except against green titans, as her huge potential-def is worth sacrificing Wilbur’s extras.