I thought I had read somewhere that Latisse was a weaker solution than Lumigan, thereby decreasing the risk of change in iris color -- though the fact that it is put on the lashes and not directly in the eye should achieve that. However, it appears that both Latisse and Lumigan are .03% bimatoprost. See http://www.latisse.com/pdf/LatisseProductInformation.pdf and http://www.rxlist.com/lumigan-drug.htm
On the other hand, Latisse comes with a set of "specially designed" sterile applicators, which Lumigan does not. I suppose there are work arounds for that but this fact, together with other good reasons enumerated, seem to make it unlikely Lumigan will be substituted for Latisse.