MDW has better accesibilty to the runways and if you stand on the corner of 63rd and Cicero you can take pretty good shots of planes in the air too! Also on the corner of I think 54th or 55th and Central you'll have a good view of Runway 31 and Runway 13.
I haven't really fully explored all of the spotting sites at MDW but it seems pretty easy to navigate around the place.
Of course, you won't any heavies at MDW (compared to LAX) other than ATA's 757, which you'll probably see one of per hour.
Its better to be on the ground wishing you were in the air than be in the air wishing you were on the ground. Fly safe!