How do I decide the electromeric effect on pent-2-ene? There are two possible case
π electrons shifting to C2
In this case, nucleophile attacks C3
It is stabilized by +I effect of two ethyl groups
π electrons shifting to C3
In this case, nucleophile attacks C2
It is stabilized by +I effect of a methyl group and a propyl group.

How do I decide which one is more stable, and ultimately, which one is favoured?
Here what you should consider first is hyperconjugation, rather than inductive effect (a brief explanation by me).

You're right that $\ce{Nu-}$ will either attack on $\ce{C-3}$ or $\ce{C-2}$, and hence two carbocations would be formed. Now, we'll analyse the stability of these carbocations on the basis of hyperconjugation which is as follows,


carbocation at $\ce{C-2}$ will have a total of 5 HC structures
carbocation at $\ce{C-3}$ will have a total of 4 HC structures


Based on the above difference, it is easy to conclude that $\ce{Nu-}$ will attack on $\ce{C-2}$ carbon. Whereas based on inductive effect, those carbocations are nearly equally stable.



Note: HC is short for "hyperconjugative structures".