Indian Wells is generally calmer, dry air, and easy to go for your shots. The surface, though, is dreadfully slow.
In Miami, the surface is faster. But it is always windy in Miami, and rain is never far away. The humidity fluffs up the balls quick, and you can't really hit your hardest all time, you will make too many errors. So even though the surface itself is faster, on tv, it looks slower.
I've played on both, IW in 09, and Miami in 07, but I dunno if they have changed since then.