Emolga may be another Pikaclone, but out of all the Pikaclones (and Pikachu, not counting Raichu) it has the highest base stat total. Not only that, but it gets an immunity to Ground (with the exception of items/attacks/etc. that ground it). In addition to that, it's speedy enough to pull off Volt Switch AND U-turn to help beat the crap out of tough Pokemon.
Derpfisk, on the other hand, does have a higher base stat total and can use Rock attacks against Emolga, but it has more weaknesses (most of the types it's weak against are also very common in-game) and is rather slow. On top of that, it can't learn Volt Switch and use its Ground moves against Emolga too easily. And if pitted against a Motor Drive Emolga, it can't use ANY STAB moves against it with a chance of success. At the very least Emolga has Flying type moves, though those aren't too effective against Stunfisk.
Oh, and Emolga is much cuter and has a better design (I like its design most of Pikachu and the Pikaclones~). Also Batmolga comic Kaori got her avvie from.