Both types have advantages and disadvantages and there really isn't one type that is overall "better" than the other; it depends on the person using the gun and the purpose for which it is used. Because my main interest in handguns if self-defense, I tend to look at the advantages and disadvantages from that perspective.
Advantages of a DA Revolver:
Less sensitive to ammunition specifically ammunition of varying power levels
Consistent trigger pull which is generally smoother than that of a DA auto
Can be fired repeatedly at contact distance without risk of being pushed out of battery
Shrouded or enclosed hammer models can be fired repeatedly from under or within a garment
Nearly immune to malfunctions induced by a weak or improper grip: if you have enough strength to pull the trigger, you can fire the gun
No magazine to be lost or damaged
More easily and quickly loaded with loose cartridges
Misfires are generally remedied by simply pulling the trigger again
Can be left fully loaded with all or most of its springs at rest
Less sensitive to cleaning and lubrication thus more tolerant of neglect
More widely available in powerful magnum cartridges
Available with better size-to-power ratios than most semi-autos
Generally have more rounded contours which many people find more concealable
More tolerant of miniaturization in regards to reliability
Simpler manual of arms is easier for novice or casual shooters to understand
No need to disassemble/reassemble the gun for routine cleaning and maintenance
More consistent manual of arms between different makes and models
Wider variety of aftermarket grip sizes and shapes due to the gripframe not being constrained by the size and shape of a magazine
Generally more accurate than the majority of locked-breech semi-autos
Advantages of a Semi-Automatic
Higher capacity
Faster and easier to reload if pre-loaded magazines are available
Generally slimmer which many people find easier and more convenient to carry and conceal
Better power-to-recoil ratio
Wider selection of makes/models to choose from
Lower price for comparable quality and condition
Available with SAO triggers which some people find easier to master
Fewer entrances for debris make it more tolerant of outright abuse
Easier to affix a light and/or laser
Better availability of aftermarket parts and accessories
Spare magazines are generally easier and more convenient to carry than speedloaders and moonclips while easier to use than speed strips or loose cartridges
Notice that I did not give an overall reliability advantage to either type. This is because which type is more reliable depends on the circumstances. If you're going to be using the gun in extremely harsh or dirty conditions, then the semi-automatic with its fewer, larger moving parts and more closed design will probably be more reliable. However, if you're going to leave the gun loaded for long periods of time without cleaning, lubricating, or otherwise maintaining it, particularly if you're not loading it with premium ammunition, then the revolver is more likely to be the most reliable option.
Advantages of a DA Revolver:
Less sensitive to ammunition specifically ammunition of varying power levels
Consistent trigger pull which is generally smoother than that of a DA auto
Can be fired repeatedly at contact distance without risk of being pushed out of battery
Shrouded or enclosed hammer models can be fired repeatedly from under or within a garment
Nearly immune to malfunctions induced by a weak or improper grip: if you have enough strength to pull the trigger, you can fire the gun
No magazine to be lost or damaged
More easily and quickly loaded with loose cartridges
Misfires are generally remedied by simply pulling the trigger again
Can be left fully loaded with all or most of its springs at rest
Less sensitive to cleaning and lubrication thus more tolerant of neglect
More widely available in powerful magnum cartridges
Available with better size-to-power ratios than most semi-autos
Generally have more rounded contours which many people find more concealable
More tolerant of miniaturization in regards to reliability
Simpler manual of arms is easier for novice or casual shooters to understand
No need to disassemble/reassemble the gun for routine cleaning and maintenance
More consistent manual of arms between different makes and models
Wider variety of aftermarket grip sizes and shapes due to the gripframe not being constrained by the size and shape of a magazine
Generally more accurate than the majority of locked-breech semi-autos
Advantages of a Semi-Automatic
Higher capacity
Faster and easier to reload if pre-loaded magazines are available
Generally slimmer which many people find easier and more convenient to carry and conceal
Better power-to-recoil ratio
Wider selection of makes/models to choose from
Lower price for comparable quality and condition
Available with SAO triggers which some people find easier to master
Fewer entrances for debris make it more tolerant of outright abuse
Easier to affix a light and/or laser
Better availability of aftermarket parts and accessories
Spare magazines are generally easier and more convenient to carry than speedloaders and moonclips while easier to use than speed strips or loose cartridges
Notice that I did not give an overall reliability advantage to either type. This is because which type is more reliable depends on the circumstances. If you're going to be using the gun in extremely harsh or dirty conditions, then the semi-automatic with its fewer, larger moving parts and more closed design will probably be more reliable. However, if you're going to leave the gun loaded for long periods of time without cleaning, lubricating, or otherwise maintaining it, particularly if you're not loading it with premium ammunition, then the revolver is more likely to be the most reliable option.
