I was worried about the possibility of damage to styrene models caused by thermal expansion but, as yet, I've not experienced any problems. Living, as I do, in a sub-tropical climate, we experience quite a big range of temperatures, exacerbated by the fact that my models are stored in a metal clad shed. Temperatures in the shed on a hot day often exceed 40 degrees Celsius, yet my Model Slipway "Sentinel", which has lived in the shed for more than five years, shows no ill effects, so far.
As for which material is better, as Colin said, it's really a matter of personal preference. I normally use ply, but in the case of my Armidale Class patrol boat I used styrene for the superstructure, as I felt it better represented the aluminium of the full size boat.
Peter.