I'm undecided on what kind of luggage i really need to do this kind of trip.
The trip notes say no xxl bags and backpack or rollers are fine but no suitcases. This is leaving me with too many choices and i don't want to feel frustrated having taken too much or too little with me.
Backpacks are tiring if worn for long periods of time and rollers are awkward if in crowds or tackling steps.
I'm 30, fit and healthy and don't suffer from back pain other than knots caused by tension from work.
I will suggest you go with backpacks. You are physically fit, so it is just fine... Visiting the nice beaches and resort in Croatia is a lot of fun and having backpacks can give you easy access on travelling.