Servine - Serperior:
Servine is kind of OK. His speed, special defense, and defense are OK. His special attack and attack are, meh, with a stat total of 413. When he evolves to Serperior he gets a bit better. He has amazing speed, and his other stats are pretty decent, not the best though. Serperior has a stat total of 528.
Cottonee - Whimsicott
Cottonee is all around BAD. His stats are terrible. He has 27 base attack. 60 base defense. 37 base special attack. 50 base special defense, and 66 base speed. Cottonee has a stat total of 280. Whimsicott gets slightly better, his attack is still pretty bad. His speed is very awesome. His other stats are pretty decent. It kinda flows the same way with Servine and Serperior. He has a stat total of 480.
So lets compare the two. Both are very fast and have OK stats. But if you look at Serperoir, he has slightly better all around stats, and a higher stat total. So I would have to recommend a Serperior.
I hope this helped!