There are a lot of factors. I used to fly LGA-IAH instead of EWR-IAH when I had to connect at IAH anyway and when I was leaving from work in the city.

If everyone was like me, you might find that LGA-IAH is better for upgrades in the morning but not in the evening. Alternatively, you might find the opposite on the return (or not, as people may be returning home to New Jersey rather than heading in to work even if their flight lands in the early afternoon).