As a general rule, I find it safer to book a late flight and then, if my business is concluded early, stand by for an earlier departure. This way, I am at least assured of getting out of town (or back to my home town!). Most of my travel is on restricted fares, BTW.

Often my later flight is during the evening rush hour, so by ticketing and upgrading on that flight, I already have my upgrade which would be far less likely if I switched to the "busier" flight same day.

On the other hand, if I booked the earlier flight, hoping to conclude my business early, and was delayed, with the load factors these days, there is an excellent chance the later departure would be full and I'd spend a night at the airport hotel!!!