The thing is how much will you need to use the dictionaries? And to what end?
If you want to look up a word and have a translation, the Kobo's dictionaries are fine. I use them almost everyday that way. If I can't find a word, most of the time I have a good idea of its meaning with the monolingual dictionary (English, Spanish...). When that's not enough, I'll use my smartphone or a paper dictionary if I'm at home. That doesn't happen often, though.
If you want many example sentences, more details or to look up less common words, I guess the Kindle's ones are what you're looking for. Once again, I've heard many praises, though I've never used one myself
If you have the opportunity to go back to that store, maybe you can try and search the same word on the Paperwhite and the Aura to see which one feels better to you.