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Daydog is probably better just for the higher speed.
The thing is both these Pokemon need some work. First off, why do they have a Sp. Attack stat at all? They should take 20 points out of that, put 10 points in Daydog's Attack or Speed stat and the other 10 in any defensive stat.
Nightdog should get +10 in both defenses, +10 in Attack and Speed, or some combination. As it is, they have decent stats but ONLY decent and need a slight edge.
Daydog almost needs to run Swords Dance or it gets forced out by everything but then in order to run that and its signature move it has to give up coverage and ensure it will get walled by something. It also needs Earthquake.
Nightdog would benefit from a Dark typing just to get STAB on Sucker Punch and Crunch and give it that over Daydog. Its speed is OK for current OU but when the Bank is legal it'll be shat on worldwide unless it's running the Double Dance set, in which case it doesn't get anything stronger than Brick Break or >Reversal and doesn't get EQ. Right now its competitive niche is basically Scarfed No Guard Stone Edge, which is good, but exceedingly limited. Why doesn't this thing get Close Combat, which would fit in perfectly with its theme?
tl;dr Nightdog is better if you slap a Scarf on it because it can actually take a hit but Daydog is better all-around although to use it to its full potential you have to run Swords Dance on it and treat it like a worse SD Weavile.
