In reply to The Lemming:
I tried a Prism and also a Rab Generator when I was hunting for a lightweight insulated top, and much preferred the fit of the Rab. You hardly feel you're wearing it compared to the Prism. Both were 'mediums', but the Prism felt a bit baggy compared to the Rab, which didn't feel snug as such, it just felt perfect. The arms on the Rab had a better length too. (I'm 6' and about 70kg/11st.)
The Rab also packed down slightly smaller than the Prism, if that's any help, but that's maybe due to the absence of a hood (which I believe doesn't bother you, based on your Alpkit Filo description) and the Prism having a larger stuff pocket.