Short Answer: Regieleki is much more viable by elimination of Regidrago.
Long Answer: Regieleki has about 5% usage in Ubers in March, while Regidrago isn't used enough to even be mentioned. Regieleki also has Ubers sets on Smogon while Regidrago has none. Let's investigate this a little though.
Regieleki: Regieleki 200 Base Speed to take advantage of, beating the entire unboosted Meta. This allows it to run a multitude of sets. One of these is a Offensive Hazard Clearer and the other being a Electric Terrain Abuser with Rising Voltage. These actually make Regieleki somewhat usable, being able to damage the opposing team with a 140 Base Power Rising Voltage. Both of these sets work for Regieleki to be usable on teams that want a Terrain Abuser or just a Hazard Clearer that can do damage.
Regidrago: In Ubers, Regidrago is outclassed on the Special side (If you for some reason wanted Dragon Energy) by Eternatus with a Life Orb. Eternatus has superior stats and coverage, so it's better in every way. Now, on the Physical side, Regidrago runs Dragon Dance sets. In Ubers, this is severely outclassed by Zekrom, who has higher Attack and Speed, as well as having Substitute for protection. Regidrago is also abused by the faster Dragon types in the tier as well.
So, there is your answer. Regieleki is more viable, part because it can be used in Ubers and also due to Regidrago's inferiority.
