The practical difference
Sildenafil is commonly used as needed with a shorter treatment window. Tadalafil can be used as needed and has a longer window, or may be prescribed daily.
Use-case questions
How much advance planning is acceptable?
Would a daily schedule improve spontaneity or create unnecessary cost?
Do food, side effects, health conditions, or other medicines affect the choice?
Which option is affordable at the prescribed strength?
There is no universal winner
The appropriate drug and dose depend on medical history, other medicines, side effects, response, and preference.