I've covered this all before, but Nasri has shown he can be a success in a 4-4-2 which we predominantly play in the league, whereas Sneijder's best position is clearly in a 4-5-1 in Rooney's role. Nasri can tackle, has outstanding technique and awareness and is the creative passer that we crave. He's not shy to putting his foot in either.
Also, Sneijder has stamina issues in a 2 man midfield that were displayed at Madrid. Great player, just not suited to us.
I think Nasri's a better player than Modric quite frankly, although not significantly by any means. He'll also get you more goals, is more versatile and is a younger and significantly cheaper option with room to improve. How can you say no?