Yanmega: Yanmega's stats are pretty good, with 76 base Attack, 86 base Defense, 116 base Special Attack, and 95 base Speed. Yanmega is a nice heavy hitter when it comes to Special Attacking. It also does have a pretty decent Speed stat and it gets access to Speed Boost which makes it even faster. Yanmega's stat total is 515.
Galvantula: Galvantula is very fast with 108 base Speed. Its Attacking stats are pretty good, but not the best. Its Defenses aren't very good either with 60 in Defense and Special Defense. Its stat total is 472
So, who would be better? Well, Galvantula is fast but his Defenses are not very good, and Attacking stats are ok, but not the best. Yanmega hits very hard with that great Special Attack stat, and even though Galvantula has a bit higher speed, if you can stall with Yanmega and keep getting speed boost to activate, it can easily outrun things. So overall, I would recommend Yanmega!
I hope this helped!