Forget eardrum-crunching summer blockbusters like “Transformers 2” and “Star Trek” — this season’s real winner may turn out to be a low-budget underdog. “(500) Days of Summer,” starring Zooey Deschanel and Joseph Gordon-Levitt, makes up in heart what it lacks in budget. As the quirky movie opens in theaters all over the city, we break down the relative merits (and demerits) of big budget rom-coms — and their cheaper, less conventional indie cousins.