It will be a bit lighter, those tanks you have shown are also a bit heavier than most. IV bags have their place, and work fine, I just find them to be more of a PITA than they're worth for me personally. You have to be sure to burp ALL of the air out of your bag before you run though, which usually involves standing the boat on its bow, allowing the air to rise as you squeeze the bag, then capping it back off... it can be a bit tricky to get used to, and it's the main reason I've swapped my boats running bags back over to hard tanks.