Speaking as a layman here.

I do know which one is safer, as to gain access to blood you have to gain access to a vein, likely by inserting a small tube made of [something] inside the vein, expect it to never break down and enter the bloodstream, become compromised, or be hit with an impact and tear the vein open to create internal bleeding. Then you also have to have a system to get rid of the waste blood (everything but the small amount of oxygen you want) all entirely within a closed system implanted into the arm, thigh, etc.