Hdtvkss you are correct there are more muscles involved in both push up and a bench press. The biggest differance between push up and bench press is push ups you will use more core muscles to stabilize body while doing a push up.

Muscle used:
Primary
Pecs
tri's
Deltoids
secondary
Lat's (Back)
Lats are firing when you bringing wieght to your chest or going down with a push up.

*** You can get a more effiecient work out by going down on a 5 count. The washington Redskins had great success by increased resistance on negatives. Great Strength builder.