I have heard from some of our models that Cialis works faster than Viagra, and does not cause the headaches or "blue vision" side effects that are very common from Viagra.
However, I've also heard some say that Viagra, and to a lesser extent Cialis, will get you hard, but actually seem to delay or inhibit orgasm. The models that have used one or both seem rather split; some are comfortable bringing it to a shoot because it helps, others say that the side effects and delayed orgasm are too much trouble, and they do fine without.
I did a bunch of research online some time ago about the different drugs and couldn't find much worth reading... that is until I found a report comparing the three in, of all places, fool.com (the investment advisors.) They did a very exhaustive report on all three, discussing mechanisms of action, studies, effectiveness, etc...
http://www.fool.com/news/commentary/...ry031126am.htm
Long and short, as I read it, was that Cialis was a better product, but Viagra sold better because it was first to market and had branding.
And I echo the other comments about the herbal alternatives. Not only do they not work very well, apparently, but many also contain potentially dangerous things like a combo of high-dose caffeine and ephedra or psuedoephedrin... these increase blood pressure, but probalby not much else.
Hope that helps