Hartm,
With a grape pack providing less than 1.5L of skins in contact for roughly one week, the pack should not significantly change the flavor of the23L of juice that you will be fermenting. The purpose of the grape packs are to boost the total dissolved solids more than impart a particular flavor.
I wouldn't be concerned with the choice of either pack - Cab/Merlot are blended (as Mike noted) as are Cab/Shiraz (depending on which part of the world you are in). The ultimate (read: costly) test would be to run three versions of the same kit: one straight, one with a Cab pack, and one with a Shiraz pack.
Do remember to be mindful of the additional sugar the grape pack will provide.
- Jim