That's a good Q matey.
I'd probably have to look at your chest and technique, etc. but I'd guess that the dumbells would be a better exercise. My chest exercises are:
Low incline Bench (25 deg is perfect)
Flat dumbel Press (occasionally)
Weighted Dips
Cross bench dumbel pullovers
I don't do flats myself because it gets to my injured shoulder and tends to blast my triceps as much as my chest. The Dips are very similar to declines but require more stabalisation (they are more basic and are a closed chain exercise) so I prefer them.
If you get on well with falt benches, then why not do INCLINE dumbell presses as your second exercise because they will be both at a different angle AND require a differnt technique as they are dumbells.