Amateratsu is greater than Kamui if Obito & Amateratsu user are inside of Kamui Dimension together and Obito doesn't use Kamui anymore to escape from the Dimension.
Amateratsu stops Kamui from hitting Itachi if Amateratsu is blocking Kakashi's line of sight of Itachi. Kakashi has to see what he is looking at & Amateratsu isn't invisible, so if Kakashi is aiming Kamui at Itachi , Amateratsu will block his vision and he will only Kamui some of Amateratsu will the rest of Amateratsu could continue on and probably hit him. Or he could kamui all of Amateratsu.
Plus Amateratsu spreads and continues even after Itachi lets it go. Kakashi has to use Kamui multiple times. They just are different.
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Yes Amateratsu is better sometimes. If its Itachi vs Kakashi yes .. unless Itachi has to close 1 eye to do Amateratsu while Kakashi can leave his eye open to use Kamui.. , but it depends on what their opponents can do & who the opponent is.