Mono panels are more expensive and more efficient (by 10 to 15%), while the opposite is true for poly: less expensive and less efficient. However, the gap is narrowing as technology advances. Poly is becoming more efficient and mono is decreasing in price. One factor which seemingly enables the poly to gain ground is that its cells are square as opposed to the cells of mono that have rounded corners. Thus, the poly has more surface area to generate electricity. However, because of their lower efficiency ratings you will need more surface area (roof/ground) to produce the desired amount of energy.
In the estimation of some, the panel manufacturer is an even more important consideration than panel technology. With panels warrantied for 25 years you want to select a company that is established and stable. Regarding technology, perhaps its best to say that one-size-fits-all does not apply when selecting solar panels. One must consider the project, budget, aesthetics, and the manufacturer.