Well, Luna, if you've been using this one "for years," I'd say it's about time you got it right.

It goes:
The difference between the almost right word & the right word is really a large matter--it's the difference between the lightning bug and the lightning.
-Mark Twain, Letter to George Bainton, 10/15/1888
http://www.twainquotes.com/Lightning.html
Over the yearss, the quote has become known popularly as the first way you have it:
The difference between the right word and the wrong word is the difference between lightning and the lightning bug.
~Mark Twain
http://www2.gasou.edu/writenow/myfavquotations.htm
The second version [that "makes much more sense"] seems to be a more modern day one:
The difference between the right word and the wrong word is the difference between the lightning bolt and the lightning bug (apologies to Mark Twain).
~from the personal webpage of Katie Roche
http://www.admissions.tcu.edu/freshman.asp
To paraphrase Mark Twain, the difference between the right word and the wrong word may be the difference between a fire and...a firefly.
~found at
http://www.timezone.com/wwwboard/BB/messages/139.html