Tadalafil (Cialis) : 17.5 hours of metabolic half-life (effective for ~35 hours), patent to expire in 2016 (US) and 2017 (EU).
Sildenafil (Viagra) : 3.5 hours of metabolic half-life (effective for ~7 hours), ED patent to expire in 2020 (US), already expired in EU. Non-ED patent expired in both.
Vardenafil (Levitra/Staxyn/Vivanza) : 4.5 hours of metabolic half-life (effective for ~9 hours), patent to expire in 2018.
Patents directly influence price and availability. By the way patents can be complicated and each substance has several patents. For instance a patent for sildenafil per-se may be expired, but there may be another patent still pending, monopolizing sildenafil’s use for ED treatment. Yet another one protecting the "Viagra" brand name. So in the US it’s still illegal to advertise/sell generic sildenafil pills for ED treatment but it may be perfectly legal to sell the same pill for another cause (e.g. Pulmonary arterial hypertension treatment). :)
http://www.cons umerreports.org … generic-viagra/
Marley Drug: Generic Medications & Drugs, US Online Pharmacy
As for effectiveness of each, that’s really not a selection variable as each drug is dosaged according to its effect per mg. E.g. Standard dosages are 25-50-100 mg/pill for Viagra, 5-10-20 mg/pill for Cialis, and 5-10-20 mg/pill for Levitra. They’re dosage-adjusted for equivalent effects, so potency comparison becomes a moot point.
From usage point of view, tadalafil can be preferred with its 35 hours of effectiveness.
From price point of view, sildenafil with expired patent can be preferred for obvious reasons.
From which point of view would vardenafil be preferred?
The question answers the original one.