Point one: Choose the 500. You have 2/16 (12.5%) chance of achieving the BIG-TA-DAH competing with a field 12 of whom are already known to have done worse than you in at least one contest. In the hurdles, you have a 3/28 (10.7%) chance in a field where the 27 others may all be as good as you are.
Point two: For most people the question is irrelevant, assuming the starting fields for the prelim contests in both sports are the same size, as only the top 25% of the 500 qualifiers even have the choice to make. If a given individual does, however, it reinforces the earlier advice, because it means he’s in the top 25% of 500 advancers, but says nothing about his chances in the hurdles.
I was vaguely told my standings in the local 500 were 2nd or 3rd place. That placed me somewhere in the middle of the pack. The hurdles competition is unknown and that pesky extra round is muddying up the field by making the final 7 competition even stiffer. So, it's either take a chance at the unknown hurdles or go in knowing others are faster than you in the 500.