Some sources claim that hand-beaten egg whites acquire more loft than mixer-beaten egg whites. We do not find this to be the case, and here in the Test Kitchen, we always beat our egg whites with an electric mixer.
There is one advantage to hand-beating, however, and that is you are much more likely to stop when your egg whites reach the consistency you are looking for. Mixers work so quickly that it is possible for the egg whites to change from soft peaks to stiff peaks before you know it happened. Check your egg white consistency constantly while beating with a mixer to avoid this problem.