The main difference is that the Paratilapia polleni has much smaller spots--this becomes more apparent as it gets older (and, thus, the appelation "pollen"-i). The P. bleekeri has larger spots, maybe 2mm-3mm. Bleeker was the man who "discovered" them in the Malagascy rivers and took them out of country.
They vary in growth time; some are slow (like mine!), I've heard of some growing large fairly quickly. My guy is about 2.5 yrs old, and is 8 inches. Started developing his smallish nuchal hump a year ago. Wonderful fish, lots of personality, selectively aggressive--he beat the crap out of my Oscar. Now he lives with one Taiwan Reef male, a baby TR, and a female JD . . . oh and a rainbow shark. He chases everyone but the JD. From what I've read from a group who are really into Malagascy fish (a Yahoo group), the Bleekeri is supposed to be less available, but that was last year, and owners probably started breeding them like crazy. I paid $4.00 for mine, he was 1 1/2 inches long.