Will say that while there is difficulty in finding the exact number, I once saw on a forum a long time ago that Armor Mode makes it so that White Fatty only takes 10% of the damage he would normally. Meaning that, if he can tank 500 Zenttatons for example, then in Armor Mode he would need to be hit with 5,000 Zettatons in order for him to be taking the same kind of damage as before.
Anyway, White Fatalis has been around longer and has more experience, as well as being smarter for having a weakness to the Dragon Element (which deals damage based on ones intellect). While it isn't clear who is stronger (Saitama does not have a specific value for his Planet Level rating, but all we know is that he is able to destroy Earth), so it isn't really clear who is stronger or more durable. However, Armor Mode will definitely aid in giving White Fatty the edge should it be pushed that far, and White Fatty also has better range overall thanks to his lightning attacks. Plus, if enough of them land, then Saitama could be inflicted with Thunderblight, which increases the chances that he will be stunned and therefore open to more attacks.
White Fatalis takes this with high difficulty, mostly thanks to better range, experience and Armor Mode.
Also for clarifications sake, Fatalis profile is old as heck and needs to be reworked. The lightning does not work around intelligence, Dragon Element does. And even if something has a base 0 resistance to Dragon Element, the Element can still hurt that being. Just thought I'd toss this out there considering that it was kind of an issue earlier in the thread.