Among Benzyl carbocation (Ph-CH2+) and Methyl methoxy carbocation (CH3-O-CH2+), why is the latter more stable?

let's see the resoance forms of benzyl carbocation and methoxy carbocation:

-Benzyl carbocation:

You are right, benzyl carbocation have multiple ressonance forms which means that It must be very stable. In fact, benzyl carbocation is one of the most stable carbocations.

-Methoxy carbocation:

In this case, we only observe 2 resonance forms.

so the question is, why methoxy carbocation is more stable despite of having less resonance forms??

Well, the key is the stability of this resoance forms...

In benzyl carbocation, none of its resoance forms have a complet octet, wich mean that the resonance forms are not very stable and therefore the positive charge is not very delocalized.

However, in the case of the methoxy carbocation, one of the resonance form have a complet octet, which means that the resoant form are pretty stable and therefore the positive charge is quite delocalized between the O and de C

In conclusion, number of resonance forms are important, but their stability is also a really important factor.