From what Suninen and others say Fiesta is best on high-grip surfaces and struggles when there is low grip.
No idea how much grip there is in Acropolis this year.

Other than that as I already commented elsewhere Polo has had more upgrades this year so far than Fiesta, but it's "final" upgrades with no more development. So while there is no doubt it's competitive this year, it might be less so in the future. So as you say it's a way to get foot in the door.