Yanmega, but despite sharing a typing, they are very different Pokemon
Here's a short summary for both
Vespiquen: the queen bee is an interesting defensive Pokemon. With three signature moves, one if which boosts its defenses and another heals it, and a solid base stat of 102 in both defenses. However, bug/flying is a VERY poor defensive typing, with five weaknesses (ice, electric, fire, flying and a painful double weakness to rock). Her decent defensive movepool, which includes cool moves like swagger and destiny bond, and her decent offensive ability are not really enough to compensate for this, but the bug still has its merits.
Yanmega: yanmega is very offensively inclined. It possesses two great offensive abilities (tinted lense, boosts the power of resisted moves, and Speed Boost, which, well, boosts its speed every turn.) It's offensive movepool is not that great (air slash, bug buzz, Giga drain, you turn, ancient power, solar beam, psychic), but because of tinted lense, it will only need its STABs to hit things effectively with its good base 116 sp atk stat. Base 95 speed isn't too bad either, and it can be made faster with either speed boost, tailwind, or a lucky boost from ancient power. Add on a handy support movepool including hypnosis and whirlwind, and you have yourself a cool Pokemon.
Here's a set I always liked (for in game)
[email protected] beak/silver powder
Ability: tinted lense
Psychic/ancient power/ Giga drain
With tailwind Yanmega (and its teammates) become temporarily faster. During this time you may proceed to beat your opponents with Yanmegas unresisted STAB. The last moveslots is just filler, but each move has its uses. Psychic is a nice neutral and powerful move, ancient power may boost Yanmegas stats while hitting fire and flying types, while Giga drain heals yanmega and hits rock types.
In summary, I prefer yanmega, but vespiqueen still has its merits, so don't discount it.