Sudafed doesn't have a rebound effect, nor is it addictive.
Oxymetazoline, the active ingredient in Afrin, DOES have a rebound effect, and it can be considerable. The drug works by causing intense vasoconstriction in the nasal mucosa (the lining of the nose) which makes the tissue shrink. When the drug wears off, the tissue has been starved for blood supply, and elaborates substances that cause vasodilation, making the swelling worse. The user then feels more congested, and is tempted to use the drug before the 12 hour period that is recommended, or in higher doses. This causes a nasty spiral of worsening congestion and accelerating drug use, so common that it has its own name: rhinitis medicamentosa. Withdrawal of the drug is uncomfortable and may require steroids. Afrin should be used in the smallest doses possible, at the longest intervals, and for the shortest time. It does not, however, usually produce the same tremulousness, tachycardia, or anxiety that the systemic decongestants like pseudoephedrine can.