A thing to think about is the destinations, Slovenia would be good for a warm up, depending on the skill level of the group, and then Italy would push the group regardless of skill level.
France apparently can be good for all levels, and UL are going out there this year for their whole trip, changing it up from Italy/Slovenia/Italy which they've done in previous years, but I'm not sure if the levels will be there in May.
I've never been to Slovenia, I know Italy quite well, and I've only really done one river in France, but that was a bit of a blur of a clusterfuck