The two choices for Pompey are as follows.
Remain in the East and build an army. Gain 5EP and 150 Denari, but lose 5NM. Loyalty drop of 25 in all of Hispania. Gades, Massalia and Hispalis no longer objectives. 15 new Oriental provincial Legions added to the force pool to build.
Go to Spain and gather your veterans. Loss of 5EP but gain of 5NM and 50 denari. Loyalty gain of 25 in Hispania but loss of 25 in all of the East including Greece. Hispalis, Gades and Massalia are now more valuable objectives.
No Oriental provincial legions can be build. The Spanish forces will be unfixed upon Pompeius arrival in Spain. As well as extra EP and Moral bonus.
Problem. The Spanish legions are already unfixed. All of them. So if you chose Spain, you gain no Legions that you didn't already have access to. All the while you lose 15 Asian legions for recruitment.
If you chose the East, you get 15 additional legions for recruitment. Spain is no longer an objective yet you have all the Spanish Legions unfixed, and you also get more wealth from the East.
There is no logic of choosing to to Spain. Is this working as designed, or were some legions supposed to be fixed until attacked. They are all unfixed after a few turns anyway.
Quick edit... I tried both options, the Spanish Legions will lock if you choose to go east. But all you do is Send Pompy to Spain and they unlock again. So I still don't see the point of choosing Spain over the East. by choosing east, you get Spanish and Asian Troops and more wealth. it's a no brainer to go historic choice, unless you want to try to force Ceasar to burn time Marching through Spain where your best Troops are already waiting for him.