Auto is good for tracking the enemy movement while Burst is good for someone who's standing still. Burst cannot be controlled, it just going to go where ever the first shot was aimed at.The enemy would have most likely be strafing and dodging your shots while shooting you.
Double Tap is good though against people caught reviving, healing, or looting if you can hit them in time. My favorite thing about double tap is the one shot against people who have 100-125 health (126 damage if all connect). Hardly worth doing though but enjoyable when you get them.
I prefer Auto the most though, especially with a purple bolt.