For us, it just all felt completely different. The Iona cruise was to the Norwegian Fjords and we splashed out on a balcony cabin. We were blessed with glorious weather (we went during the Easter holidays) and it was off the back of such an amazing holiday high that we decided to book another.
The cruise on Ventura was in the summer holidays and it felt very 'summer holiday.' The pools were rammed and a lot of parents were just off drinking all day and leaving their kids to their own devices. Clearly it was a 'different kind of holiday' and we were totally open to that. All our kids wanted to do was be in the pool too!
The ships though are completely different. Ventura felt very 'tired.' We realised it wound't be as 'modern' as Iona but bits of the pool were coming off, parts of the deck had holes in it, the food was as good but not as much choice. The big difference was how dark it felt. Iona felt bright, light and wanted you to see the sea. On Ventura, we struggled to find anywhere inside where we could just watch the world sail by. The staff on both were amazing though and all of them deserve praise for working with masks on and having to deal with 'brits abroad.'
Maybe I am being overly critical and maybe we just lucked out on our Iona cruise. But for, us, they felt like two totally different cruise lines yet they were both P&O we paid pretty much the same price.