No doubt he would pick Dean and Sam over heaven and being an Angel if it meant saving and keeping them from going back to hell for eternity. He has already rebelled for Dean and Sam but mainly Dean so I can see him making and taking the final step of sacrificing what he knows and stuff to become mortal Cas to save his brother(s).

I don't think Castiel would give up absolutely everything for Dean and Sam. I know he's done it before but I think he might think twice about doing it again.

I just have this feeling that if he knew that Sam and Dean (especially Dean, because while he did try to help get Sam's soul back he wasn't really willing to give it all up just for that) were going to spend eternity in Hell he would do all in his power to help. I don't know if giving up being an angel would really be profitable in that regard, but I do see him risking as much as was possible to help. Like I said, especially if it is Dean, because I think he really is the closest person Cas has to a best friend, and that means something to him.

I'm not entirely certain, and if the war in heaven is at a crucial point at the time, he might not. Otherwise, he might be willing to give up, if not all, then almost everything to protect his adopted brothers.

I agree with shomill, and Cas loves the guys but this season him being in the war upstairs has him changed a bit back to old angel days, like late season 4.

They're his family