According to wiktionary.org they are synonyms. However, most words have a slight difference in the way or in which context they are used. I would like to know those differences.
For example, when one has two mathematical models that describe data where you have an error measure to see their performance. Would you rather say A) or B) or is it really synonymous?
A) The performance differences of model alpha and model beta are negligible.
B) The performance differences of model alpha and model beta are neglectable.
Best Answer
I wouldn't use sentence B. Negligible means small enough to be ignored. Neglectable is just too archaic, and suggests, to me at least, a piece of fruit you could safely ignore by leaving it on the ground to rot.