Both the types of ovens can be used for heating but have very different purposes. Ovens are good for any cooking that requires baking or browning of food. A microwave does not bake or brown properly but is good for re-heating, roasting and almost all other kind of cooking. Microwaves are a good replacement for conventional cooking rather than to all types of cooking requiring ovens (like baking).
Convection mode microwve is good, as it servea the purpose of simple microwave and oven ( for baking ) too. I own one, and do a lot of baking in it