15:32 May 26, 2011
I would say "существенный" has a slightly different and stronger meaning than "значительный", though they are synonyms. I would use "существенный" when speaking about the statistically relevant change which has already gone a bit deeper and touched the essence of an event / process. (As the word is derived from "существо", i.e. essence). When you say "значительный" you also refer to a significant change but this time the main focus is on importance ("значить", i.e. signify) of this change for some external factors / groups / etc. To my opinion, "существенное увеличение" is a "substantial increase", though "significant" is also a good translation. It depends on what you want to say. | 
peer agreement (net): +8