The best medicine is always the one that works for you. People aren't all alike, that is why there are different kinds. In the pantheon of globally recognized brand names, Viagra is right up there with Coca-Cola and Rolls-Royce.
I found 12 studies — none of them funded by the drug companies themselves — in which men from various countries tried both drugs and then reported which one they chose to continue. It was a shutout: Cialis 12, Viagra 0. And the 8,300 participants favored Cialis by a wide margin — 3 to 1, on average. In studies that also included the third major erection drug, Levitra (vardenafil), Cialis was still substantially more popular than either competitor.
Women also prefer Cialis. In four studies where couples used Viagra, then Cialis (or vice versa), the women strongly preferred the latter.