Strasbourg St Denis is pretty seedy (street whores, anyone? I'm not kidding). Unless you want an experience that is a lot more hardcore--excuse me, "gritty and authentic"--than you're probably hoping for, I would go with Les Halles. But don't take my word for it--look here:
http://www.footprintguides.com/Paris/Les-Halles-the-Marais-and-the-Bastille.php"Rue du Faubourg St Denis
75010. M Château d’Eau, Strasbourg St Denis.
This is not picturesque Paris in the traditional sense, this is the real deal. No romantic buildings, just hard-working immigrant communities. At the Château d’Eau end, the rue du Faubourg St Denis brims with Indian grocers and sari shops. Detour to Passage Brady for the highest concentration of Indian restaurants in the city. Towards the southern end of the street, around Strasbourg St Denis, you’re more likely to find your appetite whetted by Turkish pizzas and kebabs. Further still, into seedy rue St Denis, prostitutes, sex shops and fast-food joints line the road as far as Les Halles."
I know they say "as far as Les Halles," but the street that website is referring to doesn't extend all the way to Les Halles, and in any case the further south (towards Les Halles) you go, the less seedy it gets.