Well, Jirachi is faster than Metagross. However, there are things in both of their favor.
Metagross: With the powerful Meteor Mash attck, Metagross is a force to be reckoned with, especially when taking into acount the high attack stat. It can also know the powerful Earthquake attack. Lower Speed is not that much of a problem with Metagross, it has Agility and Bullet Punch. It also has a higher defense stat than Jirachi. Clear Body is also a useful ability.
Jirachi: Serene Grace is a great ability, doubling the chance of a secondary effect, which can work very well with Thunder. Doom Desire is a powerful move, although I think Meteor Mash is better.
My opinion is that Metagross is a better Pokemon, and with Bullet Punch or Agility (Metagross has the defensive stats to set this up, and Clear Body makes it so that the foe is hard-pressed to get rid of it) it can be made to fit your battle style.