Both are symptoms caused by a ring-like foreign body being placed on the penis and then not being able to withdraw, only the degree of damage varies.
In penile strangulation, it just affects the venous blood return to the distal part of the penis strangulation and the penis is swollen and painful.
In penile strangulation, the blood supply to the arteries in the distal section of the penis is also impaired and tissue ischemia and necrosis occurs.