Hi Michael,
Since your input to the model is a video data, it is appropriate to use 3D image datastore. Also Unet archtecture you intent to design will be a 3D architecture and in that case going for 3d imageInputLayer is prompt.
When we deal with high dimension data, It is always good choice to go with ".mat" data storage. In particularly your case, you can write a custom function in @ReadFcn property of datastore to read the ".mat" file.
I hope this helps you,
