The only difference between them is the dimensions. We always recommend choosing the The Carry-On Max unless you routinely fly small commuter jets or you're looking for a compact carry-on.
The Carry-On conforms to the official sizing guidelines of most major airlines. If you mostly fly outside the US on non-US airlines, then we recommend this size.
The Carry-On Max is an inch taller and an inch wider and still fits in the sizers and overheads of most domestic and international airlines (excluding small commuter jets).