Oddly, The Plant List has entries for both names and seems to treat them as different entities.
It says, for instance, that "Genus Buddleja is in the family Scrophulariaceae."
http://www.theplantlist.org/tp...
On the Buddleia page, it says "Genus Buddleia is in the family Loganiaceae."
http://www.theplantlist.org/tp...
It lists only two accepted species for Buddleia (americana and davidii) and many, many more for Buddleja (including americana and davidii).
If we decide to use one (and I'd vote for Buddleja in that case), the other couldn't be listed as a synonym because it's only an alternative spelling. It would have to be listed as an aka or as an "also sold as" name.
