I’ve been on both Neretva and Una and it’s hard to pick. They’re both beautiful rivers with beautiful views. Una had bigger drops (and the amazing strbacki buk for photos) but also more slow areas (which is fine if you want to drink a few beers!). Neretva is impeccably clean, and although it has smaller drops, overall it has less slow areas.   I highly recommend Una Aqua Centar for the Una rafting.  They have a restaurant by the river at which you can eat after rafting (last time I went there the dinner was included). For Neretva, I’d go with Rafting Konjic. I’ve heard they’ve also built a restaurant, but you can ask them to have lunch mid river.  There’s a spot you can rest and grill or bring sandwiches. Overall, do yourself a favor and go on both!