If I wanted a mythical that I don't have is it better to get a mythical box or a gold box on sale?

Gold Box during the sale.

Though I wouldn't recommend spending money on the game. But if you feel like it's worth it or the game earned it, then by all means.

Mythical Boxes are guaranteed.

On average gold boxes will be much better value, but if you're very limited in budget mythicals may be safer. If you get a few gold boxes and arent lucky, you may not even get a legendary.

Mythicals worst case scenario you get the least useful mythicals - which are still OK items.

So yeah, decide what's more important for you