Tertiary carbocation is more stable than benzylic carbocation due to the following reasons:

1. Hyperconjugation: Tertiary carbocation has more number of alpha hydrogens which participate in hyperconjugation, stabilizing the carbocation. In contrast, benzylic carbocations have fewer alpha hydrogens and hence less hyperconjugation.

2. Steric hindrance: Tertiary carbocation has bulky alkyl groups which prevent the approach of nucleophiles, making it more stable. Benzylic carbocations have an aromatic ring attached to it which does not provide steric hindrance.

3. Resonance: Benzylic carbocation has a resonance structure which delocalizes the positive charge over the aromatic ring, making it more stable. Tertiary carbocation does not have any resonance structure.

4. Inductive effect: Tertiary carbocation has more alkyl groups attached to it which provide electron-donating inductive effect, stabilizing the carbocation. Benzylic carbocations have a benzene ring attached to it which provides electron-withdrawing inductive effect.

Therefore, tertiary carbocation is more stable than benzylic carbocation due to the combined effect of hyperconjugation, steric hindrance, and inductive effect.

Some authors believe that tertiary carbocation is more stable as it involves maximum +I effect and maximum hyperconjugation +H (9 alpha hydrogens).Maximum +I and +H is more dominant than +M effect. Thus tertiary carbocation is more stable than benzylic carbocation.