Very good question and either version may not work the same for each of us.
Though not a prime pec exercise, they are called into play. Probably go with the bent arm version, because of the better stretch. And with DB's, rather than a BB. Pullovers also hit the triceps very well.
Might suggest the bent arm pullover and press, for a stronger pec focus, also. A real upper body mass builder for lats, delts and triceps also.
The Nautilus pullover machine has the elbows (on pads) for better leverage and more direct focus on the lats. Even affecting the pecs much better. Good Luck.