I've used both and prefer Cialas. Why? Because it works when you want it to work. You get horny on Cialis..you get hard. Viagra...you'll be hard for no fucking reason. And the headaches and flush face are much worse on Viagra as opposed to Cialis. Cialis will do the same...but not so fucking bad as Viagra. One thing that I don't like on Cialis is the somewhat ( from to time to time ) lower back pain I get. It isn't bad...but gets annoying. I've learned through trial and error that Cialis works best in the fashion for me. They say take it 15 min prior to sex? Well the way I do it is that I take it right before going to bed LATE Friday night. That way if I get stuffy nose or flush face etc etc...it all happens in my sleep. When i wake up...it's as if nothing ever happend. I can then function all Saturday and all Sunday long. Erections come when you want them. And they come hard and strong without headaches. I used to take Cialis the recommened way and just suffered too much. Especially during sex!