The difference between Cassie and Kara is a degree wider than that between Diana and Clark; Donna would be a better opponent for Kara, especially with the Lasso of Persuasion possibly equalizing things in terms of Kara's broader range of abilities.
__________________ Why am I here commenting on this crap?
Yes. Maybe. It did one really amazing thing but it wasn't spelled out it's exact capabilities. It BRFed an enemy because she wanted it to, so it could have teleportation powers, or it could simply be a demon-banishment lasso because said foe was a demon.
Donna's lasso is better, it allows her to give commands to anyone of weaker will than her.
It's too early to say Donna's lasso is better. Not many feats for either. As far as Donna/Supergirl... Kara resisted Ares' lasso and Raven's empathy completely. Not sure if Donna would be able to out-will her.
It's still plenty useful when you consider super-science types, minions, and all the people who are powerful but not strong willed (like Effigy. GL foe who fights on a similar level, but his powers aren't based on will. Direct fight would be major hard, lasso'd be good), and it not only stops them but can turn them on allies.
In major pushed-to-the-edge situations she can probably get her willpower over some pretty strong willed people too.
And, failing that, it's still a pretty much unbreakable weapon like her old one!