Way I see it, the issue isn't really about direct comparison of QBs because what the Rams need is another QB on the roster with at least a quality starter ceiling moreso than an ability to play quickly. I mean a guy like Goff gives you both, but it's not a good assumption with any rookie QB to think they'll play above average ball. The key with QB, IMO, is having a long term view and continuing to load talent to ensure options.
I see Cook's ceiling as Phil Simms, that he's a complementary type game manager QB who can take shots and limit mistakes. Nothing wrong with that, either, as it is a proven formula for winning and meshes well with Fisher's philosophy. But the question of whether you take him at 15 is all about how you project his ceiling, not about how quickly he's going to be ready.
IMO the dude is a round 2-3 range value pick, in round 1 he's a reach. So again it comes down to how much the Rams love him, i.e. how do they project his ceiling. If they see him as a future Eli Manning moreso than a Phil Simms, they'll probably take him at 15. Otherwise, it is better for the health of the team to take a weapon or BPA there and move up for Cook at a later point.
My thing is that there are plenty of other QBs in this draft that are flawed in one way or another, like he is, who will require a little more time to be ready but who give you similar upside. And those guys can be had later, in rounds 2 or 3 as desired depending on who they are. Hogan for example comes with an ability to transition sooner, or Doughty comes with ridiculous accuracy and work ethic. You have guys like Coker with the big frame and arm, a lot of them, that need more time but who have all the measurables.
I've heard it said that this is a great backup QB draft after the top 2. I see it differently. I think it's a deep QB draft period, and the top 3 of Wentz/Goff/Lynch have very high ceilings if you hit on them. After Lynch the ceilings start dropping and it's a pick what skills you like type deal, IMO.