Welcome to the Q&A Forum

I'll give you that - it's true that the result highlighting alone doesn't prove Google is viewing "shop ipad" as 2 words for ranking purposes. The tough part is separating out why these pages rank - I've seen plenty of URLs with underscores rank perfectly well, but there's no way to prove it isn't on other factors. I think we're generally agreeing, though, that the potential risks probably outweigh what would probably be a very small boost at best. I'm also a little hesitant on having some pages with dashes and some with underscores, but that's probably just the UX side of me - it feels messy.

I would prefer 301 before canonical. This is because you don't loose as much link juice/pagerank when using 301 as you are when using canonical. In this scenario, it's best to act like Matt Cutts says in the attached video. Take a look at the vid' and please don't hesitate to ask further questions to me watch?v=zW5UL3lzBOA