iPad Pro M1 2021 (128gb) or iPad Air 4 ( 256gb)

Either iPad model will be more than adequate to download an view movies. If this is your only intended use, the M1 iPad Pro is not only very competent, but may be total over-kill.

For your described application, the only potential benefit of the current iPad Pro, over any other models of iPad, is the XDR display that is only available with the new 12.9” iPad Pro - and this only for video material encoded with HDR content. In addition, the iPad Pro includes an orientation-adaptive four-speaker array - unique to iPad Pro models.

About the Liquid Retina XDR display on iPad Pro – Apple Support

If you are downloading material for offline viewing, more internal storage is definitely preferable. As the internal storage of an iPad cannot be updated or extended, you would be well advised to purchase an iPad with as much internal storage as you can afford. It is far better to have more storage than you need, than to need more storage than you have.

More internal data storage space is always preferable.

A 128 GBs storage iPad starts out with approx.119 GBs.,after initial internal flash drive formatting.

Minus approx. 6.5 GBs for iOS/iPadOS leaves between 112-113 GBs.

Minus 3- 6GBs, for iOS/iPadOS system operational reserve, that MUST BE MAINTAINED AT ALL TIMES, to ensure/insure normal, expected operation of iOS/iPad OS, leaves approx. between 107-109 GBs of actual, usable data storage space.

256 GBs storage iPad starts out with approx. 238.5GBs.,after initial internal flash drive formatting.

Minus approx. 6.5 GBs for iOS/iPadOS leaves 131.0 GBs.

Minus 3-6 GBs, for iOS/iPadOS system operational reserve, that MUST BE MAINTAINED AT ALL TIMES, to ensure/insure normal, expected operation of iOS/iPad OS, leaves approx. between 226-229 GBs of actual, usable data storage space.

Pay the extra and get the iPad Pro with the M1 chip with 256 GBs of storage, nstaed of the iPad Air 4.

The M1 iPad Pro will, also, come with 8GBs of RAM ( which is double what is in the iPad Air ).

If you consume a lot of video content, you get the true, 4-speaker audio system that works in both landscape and portrait orientations ( the iPad Air ONLY has a true single left/right stereo speaker ONLY in landscape mode ).

iPad Pro has all of the premium features and will be future proof for a much longer period of time.