They’re two different drugs, so which one is better may come down to personal preference. Sildenafil is the generic version of Viagra and Tadalafil is generic Cialis.
The benefit of Tadalafil is that there’s both a daily and longer lasting on-demand version. There’s only one version of Sildenafil, which takes around 30-60 minutes to work and lasts for up to five hours.
Something else that Tadalafil has going for it is that the 5mg dose is sometimes used for the treatment of an enlarged prostate,
which can cause problems passing urine. So if you have ED as well as BPH, Tadalafil can be useful for treating both.
But some men prefer Sildenafil because it's normally cheaper than Tadalafil.