Both have their trade offs. Phoenix clears faster (if you aim it right), but Tiger has better early gank presence, because the DoT is really strong, especially on squishy mids, ADCs, and Supports. IMO, Udyr works better building Cinder/Phoenix with Thornmail and such, he's gets really tanky and unkillable, especially with Gromp's buff. I do like to do Tigerdyr with Devourer or Warrior, it does still work, I just like Cinderdyr better. My buddy prefers Devourer/Warrior.