Was Julia Roberts better than Ellen Burstyn?

No.

No! Ellen was far more better. There should've been no competition but someone probably bid/paid more money for Julia to win.

it's very stupid to compare two different good jobs and trying to choose which is better. Julia Roberts is amazing as Erin Brockovich. She's charismatic, strong, beautiful, in every scene and makes the movie. She deserved her Oscar. She deserved it over Burstyn as well, she was too good to beat.

I'm just going to guess that a lot more Academy members watched Erin Brockovich than watched Requiem For A Dream.

Plus Requiem For A Dream needs to be seen on a big screen with a great sound system to fully appreciate it. On a small screen it can seem more than a little over the top.

Roberts also had the cachet of being in a film that worked from start to finish, to which her performance was central, and it was a big box office hit.

Requiem can seem excessive, almost cruel. It wouldn't appeal to everybody.

I prefer Burstyn and Laura Linney of the nominees that year, as well as non-nominated Gillian Anderson, my favourite female performance of 2000 in The House of Mirth. But, really, I'm okay with Roberts' Oscar win for this role. She was going to win one at some point and this is probably as good as she will ever get. It's a fine performance.