It all depends what you are trying to do with the head unit and your personal taste man. As much as I would love to have a iihigh end SQ single din, I went with a double din because, 1) I needed built in navi cos I hate having Tom Tom's, Nuvi's, Novi's, etc. with wires hanging all over the place and 2) If you have a wide variety of music you listen to, the touch screen is hella useful when trying to find playlists, song names, artists, etc. on an iPod.
In one statement, the ease of use is what wins me with a double din. The Pioneer's are not bad at all just make sure you go with a 4V pre out unit for a stronger signal. Some of the newer models come with an 8 band EQ as well which is not too bad at all.