The reality of how similar they are doesn't really matter, not from a business standpoint. What matters is who you're trying to sell your product to. Is there overlap between the 3DS and the Vita? Definitely. However, for the most part, I don't think they're trying to appeal to the exact same audience. The style of games and services that Nintendo is going to highlight is probably different than what Sony is going to highlight, at least to a certain degree.
It's a good strategy, I think. Rather than meet the other guy head on, you come from an angle and try to carve off your own segment of the market. Sony is going to go for the hardware crowd, the tech-savvy crowd that checks out CPU specs before they buy. They're going to market brands like Assassin's Creed and CoD. Nintendo's marketing push will likely focus elsewhere.
This kind of "we're not competitors" philosophy is actually good for both companies, as they can market a different experience to people who already own another device.
