Crystal - the architetcure is built around crystal and it offers more technical depth and leverage.
I could get into a whole technical debate about why - but simply put, by going the crystal route you expand the capabilities of the tool into the development environment. I.e. you can piggyback on the architecture, by coding around the crystal delivery mechanism - take a simple case of the IDE Eclipse having a plugin for Crystal reports …
If they want pure simple out the box report viewing then Webi is good enough - but if they want to include code in and around their reports to turn XI into more than a reporting platform - then Crystal …
And to prove this - just take a jump to the left into the sap forums and see how much is being achieved on that front …