Carbocation Stability: Tertiary versus Secondary Benzyl for Hydrobromic Acid Addition
In the first instance, bromine will attach itself on the benzylic carbon as the benzylic carbocation formed in this case would be more stable than than the tertiary carbocation due to the mesomeric effect.

However in the second case, the benzylic carbonation formed would be destabilized due to the -M effect of the nitro group, like you pointed out. So bromine would favor the the tertiary carbocation which in this case would be more stable due to hyperconjugation and inductive effects.