Both are good. There is no difference between them as long as they are profitable. Fixed bidding

By choosing a fixed bidding strategy, Amazon will use the exact bid set by the seller for all impressions, without adjusting the bid based on the likelihood of conversion. This option can be selected if the seller is very sure about the bid price for the keywords or products they are running and does not want any changes in the ad bid.

Compared to the dynamic bidding strategy, the fixed bidding strategy allows sellers to get a more fixed number of clicks, but the conversion rate performance may not be as good as other bidding strategies.

Dynamic bidding

Dynamic bidding strategies allow Amazon to automatically adjust the default bid declared by the seller based on Amazon's assessment of the likelihood of conversion.

The two dynamic bidding options are "increase and decrease" and "only decrease" bidding. "Only decrease" is more conservative, which means that the seller will bid lower for ad placements that are considered less likely to convert. "Increase and decrease" is more aggressive, including the possibility of increasing bids for ad placements with a higher probability of conversion.