I've seen both used successfully on numerous High Elo streams. Vman7, in his Q&A thread, also said that good players play both styles.
-Faster initial jungling
-More AoE DPS
-Better against squishy teams
-More wriggle synergy
-More single-target DPS
-Better tower killing and ganking
-Better lategame, and against tankier teams
Lategame, when everyone has their BVeils and Merc Treads, Phoenix AoE will do nothing unless you're packin' some serious heat. Tiger, on the other hand, will do noticeable damage, especially if you picked up a Bloodthirster (AFTER your tanky items, ofc. Don't be a scrub).