The electrovalent bond, or ionic bond, tends to be the stronger of the two when choosing between ionic and covalent. The ionic bond is a bond formed between two ions that have opposing polarities, one a positive cation (+) and one a negative anion (-). The larger the electronegative difference between the two ions, the stronger the force of attraction. A good example would be the formation of a sodium ion (Na+) and a chlorine ion (Cl-). The difference between these two ions is large, sodium being from group 1 on the periodic table, while chlorine is all the way across the periodic chart in group 17. These two form a very solid ionic bond to form one molecule of sodium chloride, table salt.
It should be noted there are no "pure" ionic bonds; they all have a degree of "covalence", which means they share the required electrons between them. A good example of a covalent bond would be any of the diatomic elements, such as oxygen, O2. Oxygen has an oxidation number of -2, meaning it has room for two electrons in it's outer electron orbital. It shares two single electrons with another oxygen atom and forms a relationship between them best descibed as "codependent", hence the term "covalent".