Ibis Nantes Gare Sud is the best option.
It's in the City centre, 300m from the train station, you'll only go to the stadium once or twice over the weekend, and if it does fill up, theres a Mercure hotel even closer to the train station for pretty much the same price.
And apparently, one of the tram routes in the city goes from the centre right past the stadium.
I haven't booked yet, but I know where I'm staying.
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