It tends to be very personal so the best thing is to experiment yourself. Technically (and it won't apply the same to everyone)...
Overall...
Viagra (sildenafil) is the oldest drug and works very reliably (studies and reported opinions), quoted time about 4-6hrs.Some side effects like headache or tummy upset. Greater likelihood of headaches than other two reported
Cialis (tadalafil) is slightly newer and works for a much longer time (quoted at about 18-36hrs). Some side effects, especially dryness, stuffy nose etc. Affected negatively by food in the stomach, probably more so than Viagra.
Levitra (vardenafil) has zero side effects in practice though some are quoted as possible (similar to the other two), quoted time about 4-6hrs, tiny tablet not affected by food or anything. Works much faster than the other two.
Roughly: 100mg Viagra = 20my cialis = 20mg Levitra
Prices: Viagra cheapest, Levitra the most expensive.
A couple of studies
External Link/Members OnlyExternal Link/Members OnlyHere's another comparative study, detailed, that gets different results:
External Link/Members OnlyThere just isn't enough research to give more definite answers. My own personal preference is simple logic: if doing lots and lots (really lots and lots) of fucking and wanting a 'booster', take small amount of tadalafil well before starting and every 3 days. Use vardenafil for a fast top up if needed for faster recovery. But until you try it, experiment with different ones and dosages, there's no really telling. Every set of results is 'a percentage' - which means then a high enough chance it won't aply to you.