tnx for recommendations! To be honest I often download my fav videos from YouTube to watch them anytime I want

and with this you'll spend lesser data,really big help.

No, there won't be any changes in the data consumption from streaming and downloading so it all depend on which quality you choose while streaming or downloading.
https://forum.blu-ray.com/showthread.php?t=277790&page=8

I would close out all the other apps and not watch it in max quality