niether.. they both suck by having negative side effects on your natural testosterone levels thus making you completely dependent on them.
The very best prescription for a solid rock hard disck is to increase your natural testosterone levels ... and the best way to do that is with exorcise .. especially cardio and avoid sitting at the computer in a chair so much ( inhibits natural blood circulation )
however if you must take something:
instead try Rock Hard Weekend
http://www.rockhardweekend.com/ I swear not only does it work better this stuff will not only give you a raging hardon on command but I swear it makes your dick 3X bigger for at least 2 days.
Horny goat weed also works just as well as V or C and has been around forever.
I can take 2 horny goat weed pills from GNC and fuck man I could fuck every woman in my zip code and have enough stamina left over for a few in the next county over. I think the dosage is based on height weight. I am 236 LBS and 2 pills does the job for me if I needed it
but exercise and not sitting on your nuts all day is the best medicine