I'm surprised it hasn't be mentioned yet.... use the new Kodak Portra 400, you can push/pull that film at least two stops in either direction without needing a push/pull in development with some very pleasing results. Especally the 120 size. I've shot up to 1600 in 120 and 800 in 35mm. The colours become punchy and you get a great contrast.
Shot on a Rolleiflex 2.8F, Kodak Portra 400, exposed at ISO-1600, no push in development.
What about color slides? Which one I can use in low light conditnions and how to use them properly. Really like to shoot indoor portrets.
Provia 400x can successfully be pushed to 1600. The results are a little contrasty, but much better than you would expect.
I'm terms of real film speed as seen in shadow detail I think slide film pushes better. This was the commonly accepted wisdom at one time though that seems to have changed now. I always had good luck pushing slide film but I was doing my own E6 and got best results increasing development time a bit more than specified.
Provia 400 is a good film but already grainier and contrastier than 400 print films and both will increase with pushing.
I just had a chance to push the Portra 400 film to 800 and I'm actually quite pleased. This was in very low light so the images were less than stellar (probably my fault also) but wow... I"ll have to try them at 1600. :)