Convection ovens work by circulating hot air with a fan for faster and more even cooking. Look for an oven with dual convection, meaning the air is circulated from the left and the right.
Microwave combo ovens also may come with a grilling feature. An oven with multiple grills, an upper and lower, will provide more cooking options.
When comparing the microwave portion of the combo unit, look at the wattage ranking of each oven. The higher the wattage the faster the cooking time.
Microwave combo ovens come in a variety of sizes. Look for a size that properly fits your cooking area. Common sizes are 24-inch, 27-inch, and 30-inch widths.