The main benefit is that rails-assets just generates a gem based upon the original source code. It's nothing special but easily updated. The custom gems like bootstrap-sass sometimes lose the maintainer and don't get updated making it hard to use when newer versions come out. That said, I still usually prefer the custom maintained gem because I've had trouble with rails-assets a handful of times and can at least maintain the custom gems myself.
It's really personal preference. Not a huge benefit either way, just different takes on it.
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