im wondering, is there a difference between blurring and feathering ?
or is that the same applied in different ways ?

I am not sure just how much different they are. When you use blur, it blurs the whole image, when you use feather, it feathers just the amount from the outer edge that you set it for. I really like the feathering tool a lot.

GimpDocs says: "Note
For technically oriented readers: feathering works by applying a Gaussian blur to the selection channel, with the specified blurring radius."

So, technically, it seems to be the same thing.

Yes about selection feather and blur ( blur the quickmask or the selection channel ) are totally equivalent

But blur the quickmask (or the selection channel if was saved in a channel) give more control because makes a easier see how much the selection will be feathered

Feather selection may seems bit quicker because require 2 steps less (enable quickmask and disable quick mask, or save in a channel and do channel to selection) BUT are 2 steps that take really few seconds and allow to notice defects in the selection, expecially in partially selected area (that the marching ants display can't show) impossible to notice in other ways.