Which of the these SN1 solvolysis of two alkenyl bromides reactions is faster?
[...] SN1 solvolysis [...]


Let's recall what we know about these. We have a two-step reaction, consisting of 


heterolysis of the starting material
association of a nucleophile $\ce{Nu}$


We remember that the heterolysis is not assisted by $\ce{Nu}$, there is no backside attack of $\ce{Nu}$, pushing out the leaving group. We also remember that the association step (addition of $\ce{Nu}$ to the carbocation intermediate) is usually pretty fast. Consequently, heterolysis, i.e. the formation of a carbocation is the rate-determining step.

Thank's to good old Arrhenius, we know that the rate is determined by the activation energy of this step.

The activation energy is lower when the resulting cation is more stable.

The cation is more stable when the delocalization of the charge is better.

Distribution of charge on the vinyl cation is better when it's actually a 
styryl cation.



What is the geometrical requirement for this interaction and what might prevent it?