I chose viagra simply because it does seem to allow for harder erections than cialis. When I first started taking pde-5 inhibitors, keeping hard was priority for me and viagra was perfect.
rnch is spot on though. Cialis gives fewer sides. It is also nice not to have to worry for a few days about any minor ED issues (or too much alcohol, what effect BP meds might have etc).
My current preference is to use a bit of both Cialis and Viagra. I take 10-15mg cialis for a long weekend. When I want to be sure to stay hard as a rock for a few hours, I add my usual ~10mg dose of viagra to that. Works like a charm.