There is often great debate over whether you are better off with poly-crystalline cells in your solar panels or mono-crystalline solar cells in your solar panels.
I think this question is one of the last things I would concern myself with if I were buying solar electricity panels and here is the reason why. When 2 panels are marketed at a particular wattage (say 200 watts) at standard test conditions then in these conditions both will produce at least 200 watts, provided both of these panels have no negative power tolerance in their specifications. This is the case even though one of these panels may use polycrystalline cells and one of these panels may use mono crystalline cells.
It is true that theoretically it is possible to make a more efficient mono crystalline cell than a poly crystalline cell but in the example given above each of the 200 watt panels have enough of each type of cell such that in the same conditions they make the same amount of power. It may be that the mono crystalline panel in the example above is fractionally smaller than the poly crystalline panel that produces the same amount of power, because perhaps the mono crystalline cells used my be slightly more efficient but in the end both are going to produce the same amount of power.
A better cell efficiency simply means that the mono crystalline panel might be fractionally smaller but if you look at the sizes of commercially available poly-crystalline panels and mono-crystalline panels of the same wattage (say 200 watts) they are very close to each other in size. Both types of solar cells usually have a similar temperature coefficient and so in practicality both are likely to produce an almost identical amount of power over time.
Similarly, there is no evidence that either poly-crystalline solar panels or mono-crystalline solar panels decay at a different rate over time. This is why I would not be too concerned about whether the solar companies I talked too offered me mono-crystalline solar panels or poly-crystalline panels.