Isochrone driving/public transport map : Grass or postgis?

I have to do Isochrone driving / public transport map. It would seem that the two better open-source solutions are grass and postgis. Could you tell me which one is the better? the easier to implement?

I' m working with a windows 7 32 bits (So i am limited by the memory capacity). Last time, i made a quickly test with grass v.isochrones plugin and a error message appeared saying "out of memory"...

Regarding my network vector data, i have already downloaded OSM data.