You still think of Donna from, say, 25 years ago, when your statement would have been true.
Simon has a strength (though not what you might think) and durability advantage; Donna has a speed and combat-skill advantage. If things start to go bad for her (which they can, certainly), all she has to do is finesse a lassoing, and WM goes down; his will is not a match for hers, and the Lasso of Persuasion makes for at least a couple of her wins.
As I posted, likely a general split. Simon's *slight* majority (which, even as I think more about it, I am reconsidering) comes largely from his energy powers. But Donna can deal with a good amount of that via her bracers/speed, so that's not a huge advantage, by any means...
__________________ Why am I here commenting on this crap?
Donna Troy has upgraded her strength over time, especially post-resurrection, and during OYL filled in for Diana as Wonder Woman. She's still less strong than Diana, but she's reasonably close.
Her lasso is, in it's own way, better than Diana's in combat. It allows her to command those who has less willpower than her.