I would imagine that the sultan will actually do a better job on the algae than the tiger, the tiger is more of a carnivore.
Way i see it is the qa and the tiger are pretty similar looking species, if i was thinking about the tiger i'd rather try and get a m/f pair of the qa's to perhaps breed them, whereas the sultan is a completely different looking fish to the one you already have.
as for which one is more active im not sure tbh, plecos i get never seem to fit the 'pattern' anyway, their own personality seems to come through rather than what they 'should' be like
hth