The best tomatoes to use are the freshest, ripest ones you can find. For this recipe, I do prefer the meatiness of a farmstand beefsteak or heirloom tomato, and I think red tomatoes pair better with the red wine vinaigrette than yellow ones. But really, ripeness trumps all. If you decide on cherry tomatoes, I might chop the red onions smaller, rather than leaving them in slices.