I'm not an expert with these, but as I understand it, an alternator is a synchronous AC machine with a rectifier and regulator (usually on board). The regulator keeps a narrow voltage profile by varying the field current to the rotor. If you're handy enough to modify the field excitation, you might get usable DC at lower voltages at lower rpms. A great advantage of an alternator, besides the low cost of salvaging one, is the pulley mount, and you might get a better low speed torque profile.
DC motors are excellent for low-speed torque, but I think the inverse of that is difficulty in starting them in generator mode, especially with a belt drive.