Brussels - Warsaw : Train or plane ?

Hello,
I wan't to visit Warsaw for a week or two in August, i wonder what is the cheapest way to reach Warsawa ?
Train via Köln and Berlin or plane ?
If i take the plane , wich is the best company ?
Thanks !

Wizzair or Ryanair from Charleroi or Eindhoven. I personally prefer Wizzair & consider Ryanair a bunch of thieves. I might show you some sights if I will be in town that moment.

Thanks a lot ! Very cheap tickets !
I don't know yet when i will come , i will open another thread .