This question comes up pretty much once every two or three years, because of one man: Christopher Nolan. He's developed a bit of a brand of producing these bigger-than-life movies and a huge advocate of audiences seeing them on the biggest possible screen imaginable, and of course in 70mm IMAX.
To the devout, anything less is ananthema.
As an industry professional and member of the "exhibition" community (as movie theater operators are known in the industry), I love that Nolan is such a vocal champion of the cinematic experience. Hearing him speak and present Oppenheimer at CinemaCon reminded me why we do what we do.
At the same time, I don't agree with the over-emphasis placed on the 70mm format. With respect to Mr. Nolan, who is by every standard a master at his craft, there is much more to exhibition than the size of the film stock, and these are important considerations that help form the quality of the viewer's experience in many meaningful ways. Convincing the general public that they are not getting the whole experience unless they see it in 70mm discourages people from going, especially when such presentations are very rare.
What is better about 70mm?
As others have correctly pointed out, 70mm contains significantly more picture information than any other standard format is capable of containing. That's great, but the real question is, can you see this? Perception is reality, and human biology is a much bigger limiting factor than the number of pixels in the projected image. Also, film doesn't have pixels, so it's not exactly a useful comparison in the first place.
If you look carefully, even in a 4k cinema, you'll be able to see pixelation, especially in non-natural shapes like text. It is barely noticeable, and you have to look for it, but it's there. On film, this is not a problem. So, is this the end of the discussion? Hardly. The moment the image moves, the pixelation is gone - lost to the many signal processing techniques our brains have developed to compensate for the various limitations in the biology of the eyes.
So, if we allow that a 70mm image contains more visual data, then does that mean that 70mm is objectively a better format to watch a movie? There are many reasons why it might not be, all of them practical, and all of them more important to quality than the number of pixels on the screen.
Additional Picture Quality Factors
Resolution is one of many measures of picture quality. Others include brightness, contrast, stability, illumination uniformity, and color range and accuracy. It should not surprise anyone to know that there are SMPTE standards governing all of these aspects and more. As exhibitors, we calibrate our theaters, big and small, to these standards. IMAX and Dolby Cinema, among other premium formats, have expensive technology to exceed these standards.
Digital cinema projectors were specifically designed to meet these standards with much greater consistency than film projection equipment, and with low or no training required from the cinema's staff. This is huge, because it helps ensure that digital presentations of all varieties run better and produce a significantly better product than the average film presentation.
Overall, film really can't compete with digital in these areas. You're going to get higher color accuracy, better (perfect) stability, excellent illumination uniformity, and better brightness with a digital presentation. All of this is due to the stubborn laws of physics and how a digital image is produced.
Additionally, film degrades significantly as it's run. Even after one day of showings, a film print will accumulate dust and scratches. By the end of the run, it is quite common for a film print to be unusable due to the accumulation of damage. And this damage is much easier to see than pixels on a 4k digital image. The size of the 70-foot Imax screen makes it even easier to spot these issues.
Other Quality Factors
We've so far failed to discuss one half of the cinematic experience, and that is the audio presentation. Thankfully, we don't see Mr. Nolan out there extolling the virtues of optical stereo. We can all be thankful that uncompressed digital audio is the standard for all digital cinema presentations. 70mm Imax also uses a digital soundtrack provided on optical media.
And, while Imax theaters are particularly known for their sound systems as well as the large screens, Dolby is another company that has operated in the cinema sound space for decades. Their audio formats, including Dolby Atmos, contain more channels and more stringent calibration requirements than Imax. Having regularly attended movies presented in both formats, it is my opinion that Dolby is the superior format at present.
Should you endeavor to see Oppenheimer (or any other movie) in 70mm Imax, as the director recommends? I would say no. Instead, I would recommend seeking out a regular digital Imax, or even better, a Dolby Cinema for the best premium experience. The next best thing would be to seek out a premiere house at a good theater near you. You can tell which these are because they typically have at least 15 or 20 seats across in a recliner configuration. I would avoid theaters without recliners.
Ultimately, a moviegoer looking for a memorable and high-quality experience as Christopher Nolan intended doesn't have to attend a 70mm presentation to get one, nor is it necessary to shell out upwards of $20 for a premium ticket. With today's digital presentations, there is a lot higher quality than with film even in a regular auditorium.