You can't go wrong with either.  IMO the Shift, Agent, and Gravity Shift are all in the same class with different shape characteristics.

If you plan is to play straight up the boards may I suggest the Gravity Shift instead of the Agent or Shift.  Of the three the Agent and Gravity Shift are both lower RG - which is probably favored for track bowlers due to the fact that they will give you a more squared entry angle to the pocket.  The Agent is Reactor and symetric vs. the Gravity Shift is R2X asymetric.  I like to use my Agent inside more and the Gravity closer to the track area when there is more oil.

Gravity Shift angularity will depend a lot on the mass bias location, i.e. it is more layout specific. That being said, I tend to find the Gravity is a lot better suited for straighter/track shots than the Agent.

Personally I like to have the Agent underneath the Special for me, but generally speaking I never matched up the best with the Shift series. They are very much arsenal balls for me because I find the reactions are very condition specific.