Theres only slight differences cosmetically, the taillights and the trim covering the cornering lights comes to mind. Engine wise, both had the same displacement, but the '70 had a slightly improved oiling system.
Actually, the grille on the '69 is more complex (and expensive) than the grille on the '70. The '69 had those smaller blades in between the bigger blades, making the '70 a little more "plain-looking." It's only a slight difference though.
The '69 and '70 are both similar and they are both reliable (and powerful) years for Cadillacs.
The '69 has a 10.5:1 compression ratio and the '70 a 10.0:1.
The '69 was the last year for real wood trim (in the Fleetwood series and leather-equipped de Villes).
I'm not sure if the grilles will interchange but it would look kind of strange for a '70 to have a '69 grille- you would confuse a lot of people who see it!
I have a '69 (Fleetwood) and '70 (Sedan de Ville) so I'll check tomorrow if the grilles bolt in the same place and check the sizes of the grilles.
Like I said, the '69 and '70 are very close as far as build quality, comfort, power, etc. One bonus of the '70 is that, according to those Consumer Reports frequency-of-repair charts, the '70 has the best repair record of any Cadillac (with the '69s very close behind).