In my opinion both make great starter pets. I got my bp around 7 years ago, and looking back a corn probably would have been a better fit for me husbandry wise. If you do your research and get the proper equipment both will flourish.
Corn Snake
Pros-
-More forgiving care requirements
-Great feeders
-Come in a variety of morphs
-More active than a bp
-Easy to find and purchase
Cons-
-Fairly quick and they don't really sit still (Although if you are like me this would probably be a pro)
-Smaller so they are more prone to handling injuries
-Babies can over heat in your hands
Ball Python
Pros-
-Slower snake (Honestly in the day they are kind like pet rocks, but mine is fairly active at night)
-Thicker so you are less likely to injure it while handling
-Come in a variety of morphs
-Easy to find and purchase
-Husbandry is fairly easy to get correct with the proper research and equipment
Cons-
-They can be very picky eaters
-Husbandry is less forgiving
-They go on food strikes (If you are prepared for it then it is not really a con, but a lot of first time bp owners panic when their bp is not eating)
I also agree with Bogert that a juvenile corn snake would probably be the best option. No matter what make sure you do plenty of research on the species you decide to purchase.