I have flown both engines and there is VERY pronounced difference in power when using the 1.00. For a measly 2.5 ounces it's not trivvial in the least. The 1.00 also is a bolt in replacement for the .91 so physical size isn't an issue either. Other than the price difference I really can't see any reason to prefer the .91 over the 1.00, unless you just happen to have one setting around not being used.