Oh, where to begin.......
The later SD1 was not a bad car, very roomy, and that hatch was very useful. Build quality left something to be desired however, and the materials used were not the most durable.
The 1970 car, if truely mint, would be the one that I would go for. Build quality was still way up there in the Rover tradition, and the car handled very sweetly. There were manual-transmission 3500s, (I used to own one!) however they may not have been sold over on this side of the pond. You can get the trannies from the UK still.
Things to watch out for on both cars are rust, rust, and, oh yes, rust. Mechanically, make sure that there are no signs of overheating. Not a good thing with those all-alloy units.........
On the P6 (the 1970 car) check very carefully for rust. Ignore the outer body-panels, they are all bolt-on, and are very easy to replace. Even the roof panel. With a decent paint job, a rot-box is transformed into a trailer queen! Look carefully at the wheel wells, inner wings, and especially the areas around the rear suspension mounts (don't ask how I know about the latter!). If everything checks out underneath, that is the main thing on these cars, and any mechanical parts can be replaced easily.
Personally, given the choice between the two, I would go for the older car, no question.
There are some very knowledgable members on this board who check in often, so stay tuned. Rovernut especially knows these cars inside and out.........