If the number of objects approaches the number of pixels in the screen, you would be better off with plasma.
Particles: probably not a good idea for trails, specially for objects so small
Sprites: good if there's much less objects than pixels in the screen, you'd have to do some math to get the trail looking right. Do the objects turn? if so, you'll have to segment your sprites and each segment needs more math. When comparing pixels vs objects, double the number of objects for each segment on your trail, if it's still less than pixels go with sprites.
If your objects are 1x1 px, consider using the Line object instead of sprites for the trail. Less vertex data, less computations, faster and simpler to understand. The segment thing goes for lines too, though.