Is gtx1660 or gtx1070 better?

Is the GTX1660 with 6GB or the GTX1070 with 8GB better?

For example, I would like to play Fortnite with the lowest settings with 144+ Fps (possibly also with 1440p)

The GTX 1070 is better and has 2GB more VRAM. Therefore also better suited for higher resolutions.

I don't know right now, but I do know that there are websites out there where you can compare graphics cards.

1070 is definitely better

The GTX 1070 has more power. It is also better suited for WQHD thanks to the 2 GB more VRAM.