Having just recently been to Rennes
(as it's my préfecture, I live nearby)
My advice is simple - go to Nantes. Went for the first time by train in August, and the attitude and behaviour was remarkable. I found Nantes open and friendly and vibrant. Rennes, on the other hand, is best described as shades of grey devoid of colour (though, I will grant, the métro is nifty).