electricity cost are not $0 from a net point of view.
However some countries has electricity included in the rent, also it is not unusual that student flats, dormitories ect has electricity included in the rent.

Personally I will go for the HD 7950, because of the availability where I live, you can find it used for a good price.

However I see many "pro-miners" use R290x, R280x, R270x.

I think both are good miner-cards, go for the cheapest you can get your hand on.

Good luck - happy mining