Tadalafil or sildenafil — what is the difference?

Both are PDE5 inhibitors. Tadalafil has a much longer half-life, which allows either on-demand use or a lower daily dose; sildenafil is shorter-acting and taken on demand roughly an hour beforehand, with onset delayed by a heavy meal. Side-effect profiles differ slightly. Neither may be combined with nitrates under any circumstances. Which suits you is a clinical decision based on frequency of use, other medications and tolerance.