Monel is between stainless and aluminum in strength but it is usually chosen because it is slightly lower on the galvanic scale...which means it will have slightly less of a corrosive action on aluminum. It is quite a bit more expensive than stainless steel.
Personally, I feel that the difference between a stainless rivet and a monel rivet aren't worth the difficulty in finding (and buying) the monel. However, Monel doesn't apply quite as much compressive pressure on the components being riveted - so in the case of riveting composites, I would go monel.