Although no expert on the matter, I don't think that Antonov will support the M-28, and PZL will support the An-28. I do think there is quite a difference in them, ecspecially the engines and avionics, the things were you want to be sure to have spares. The M-28 isn't very common in Africa, the An-28 is used more often, but I don't know how well its supported by Antonov or any private maintance company. Its also a fair bit larger then the PC-6 and Caravan. With the An-28 your getting closer towards the Let-410 and Twotter class.