Imo, with 35mm you will NOT get much detail beyond 1080p, and with 70mm, you will NOT get much detail beyond 4K. This is AT BEST.
Not true, but the real answer is complex.
On some of the Star Wars 35mm prints I have scanned, there is plenty of detail at 4K that just isn’t there, and in some cases, cannot be there at 2K/1080P.
If you resize the 4K scan to 1080P that detail is lost, the classic example is the door seal on the Tantive at the opening of Star Wars (Ep IV). On the 4K scans you can see the ridged detail, on the Blu-ray and any 1080P source, it is a non-descipt grey.
However, the Blu-ray, even in that same scene shows some detail not in the 35mm prints and yet is missing detail from the 35mm prints at the same time
So what is going on?
It is down to transfer. When you scan from the neg, you pick up all sorts of detail that will never make it to a print, and some of this survives the down-rez to 1080P, so the Blu-ray is more detailed in some areas than the prints. Yet there is some detail that is simply finer than 1080 lines, so can never be seen on any 1080P source, uncompressed or otherwise, yet will show up nicely on a 35mm print.
So the answer is… it depends.
There is definitely detail on the Tech IBs of Star Wars, that are well above 2K, but if your final delivery is 1080P, then it probably doesn’t matter. If you want to make a UHD/4K version for viewing on your snazzy UHD/4K TV however, then that detail can be retained if you scan at 4K or higher.
I tend to scan at a vertical resolution of 3400 pixels or so, and that seems to retain all the detail from the 35mm prints, dropping to 2160 pixels vertically definitely loses some visual detail, so I won’t scan archivally below 3000 pixels, but if your delivery is 1080P, then you could probably scan at UHD resolution, and end up not seeing any difference.
That is all talking about 35mm prints. A 35mm neg however easily can have detail that exceeds 4K, but again, unless your final delivery is better than UHD resolution, then you may not need to scan any higher than 4K full aperture.