How likely is a?  Even if it ever happens, how likely is it this utils package would contain all the same functions that underscore does?  If it's guaranteed to have 100% of the same functions then I can see your point.

For b, is _() used in javascript or just haml/ruby?  If it's the latter then I would say keep underscore as _.  These are different languages after all.  I wouldn't expect ```someArray.indexOf()``` to work in ruby.