Cialis stays in the system for 36 hours. Viagara stays in the system for 4 hours.
Any doctor will tell you they are both just as effective, but because of the timespan, it just depends on your lifestyle. For example; Im 20 and I can never time when exactly Ill be having sex, so I take cialis and Im good to good for 3-4 days. Where as Viagara is probably better for an older married fella who knows exactly what time he'll be having sex every night.
Ive tried both and personally I think Cialis is just better and way more convenient, but that is just my opinion. I can take it once and Im set for days, where Viagara I have to time it.