Alternators spin the magnetic field generating an alternating current that doesn't have to go through brushes, makes them less maintainence intensive. The objection I have with pto alternators is that tractor governors are less sensiive to load changes. Electronic equipment and motors can be damaged by slow or fast frequency. The governors on alternators with their own motors are much better at maintiaing 60 cycles.
Generators, on the other hand, make direct current which, without a commutator, to convert it, can be converted to alternating current using electronics to maintain the proper frequency. Everything is a trade off. /forums/images/graemlins/confused.gif