well lego would take up less space, but may be more frustrating if bits come off. does he already have lego? my 4 year old only plays with the figures, we built the building and vehicles, he destroys them. but we have a playmobil castle and thats one of his favourites.
lego you can save for when he is older but not the other way round iykwim so i'd maybe go with playmobil