If you are here then you are thinking about using the insane res build. While it was primarily created for those with very warped build surfaces it will nevertheless still be usable by those using surfaces that are not warped and it should not cost much in terms of time before each print. In fact it is likely to save you time before each print.
Both systems are very similar. ABL however is more suited to probing the entire bed before each print whereas UBL is better suited to probing the entire bed on a once off basis and then only doing a quick probe before a print to determine whether the bed has shifted in orientation at all. ABL could technically produce the same resolution as UBL however it would require a detailed probe of the entire bed before each print which would take forever.
Both systems create a mesh of the bed which helps Marlin to understand how to adjust the height of the nozzle as it moves over the bed.