Notes

I find that I’m of the opinion that katakana should be learned first, for a few reasons. First of all, hiragana, in practice, is a supplement to kanji. As such, unless you are reading a book for kindergarten students, you aren’t likely to see whole words written entirely in hiragana. Compare this to katakana, where you will see whole words (including company names and the like), and you can see how katakana is comparatively more useful in a certain sense.

The other issue is that Japanese learners often learn hiragana first and then call it a day, which results in low reading comprehension with katakana. This is made worse by the fact that katakana characters are often more similar in shape than hiragana, so if you are struggling to read it, you are going to make more mistakes. Thus, if you learn katakana first, you may get more practice and proficiency with it.

Lastly, katakana is used to note sounds (which is why it often gets associated with foreign words). As you start learning Japanese, you aren’t so much speaking Japanese as you are parroting the sounds. Therefore, using katakana is kind of thematically appropriate.

Like I said though, katakana characters are visually very similar to one another, so it is a little harder to learn reading. Honestly, if given the chance, I would just learn them together. In Japan (and in Canada), we ultimately teach kids the upper and lowercase alphabet at the same time, so I don’t see why a Japanese learner couldn’t take that approach. As noted by others, neither system takes a great deal of effort to learn, so there is no reason to not learn both, either in quick succession or simultaneously.