I had a similar question - why should I buy Notion for iPad, if I already bought Progression for iPad?
What would be helpful for buying decisions would be a feature comparison of both products.
On the other hand, the prices for the iPad versions are only a fraction of the prices of their big brothers, so thinking too long might waste time which you could otherwise use to create value.

I am a happy Progression user since about a couple of weeks and bought Notion as well today.
For me the reason was to have MIDI import and export and to be able to have the classical rock band setups, which would need drums and perhaps keyboards.
On the first look, I can't spot something which is possible in Progression, but not in Notion.
So far Notion seems to include the Progression features.