Yeah, there's a pricing anomaly in those plans, which makes the "$10/month" flex data plan the better of the two:

$10 for up to 100MB
+$10 for the next 150MB
+$10 for the next 250MB
+$0.02 per MB after that ($20/GB).

Expensive compared with wireline data, but probably okay depending on how much the phone uses with a typical set of apps.

Cheers