I prefer flat triggers on my vertical grip rifles, curved on my sporter gripped rifles. The vertical angle aligns my finger better to where I have even and consistent placement on a flat shoe. The curved triggers feel fine on vertical grips, but I just personally feel like the flat shoe allows for a better press. However I threw a flat shoe in a rifle with a swept, more sporter type grip, really didn't like the feel, it was like my finger was wanting to slide up the shoe. The curved gives me better placement and consistent pull with a swept grip. Therefore, I prefer flat shoes on ar's, chassis, and stocks like the EH-1 or similar. M40 type or more swept grips, I jump for the curved.
This is, of course, all personal preference. To each their own.