Dysport is an excellent choice of neuromodulator for smoothing the crow's feet lines. At least one study showed (and my anecdotal personal experience confirms) that there is a slightly faster onset of action of Dysport over Botox in this area, with equivalent effectiveness.
The amount you will need can best be determined by an experienced physician injector, as it does vary somewhat from one patient to the next. As a general guideline, I find that about 20 units per side works well for the majority of people. I also charge by the area treated, not by the unit, so that both the patient and I will not feel a financial pressure to possibly undertreat.
There are very few possible side effects from treating this area with Dysport, aside from minor temporary bruising at the injection site, so don't fear the regrets and go for it!