This is a big question on everyone's mind right now. "What stack should I use" is always a big question, but sometimes web development goes through a particularly notable state of uncertainty.
Although there are a lot of different tech stacks, I think that most of this boils down to SPA vs integrated views with some javascript. In the rails world, this generally means either Rails plus jQuery, unobtrusive javascript, coffee script, and so forth, vs rails-api and Ember (or another javascript front-end).
Right now, I lean more toward the integrated views than most of my fellow developers. Very few people would take an always/never position on single page apps, so this generally boils down more to how far people want to take them. Some are more eager to apply them in a wider range of scenarios than others.
I personally lean toward the integrated view approach, and I advice caution around SPAs - in my opinion, very much an opinion here, I think they add a lot of complexity compared to the integrated approach, and are still in a state of flux. If what you are writing really is mainly a set of forms that you'd like to enhance a bit with automatic page refreshes, drop downs, drag and drop elements, autofill and so forth, you may want to stick with a more stable stack that isn't evolving as rapidly as SPA javascript frameworks.
There are situations where you can find yourself in a real mess with an integrated view, that would be far easier to manage with Ember or another javascript framework, keeping your backend logic in a relatively simple API. Not sure if that's going to happen in a CRM - they tend to be pretty form-ish apps, but perhaps that's because up to now, difficulties with javascript have led us to think of them that way. The rapid evolution of JS frameworks may, for all I know, have opened up an opportunity for serious innovation here.
One other thing - remember that it is relatively easy to expose a rails method as an API even in the absence of rails-api. My guess is that this is true of most integrated frameworks that provide a view tier. You won't be locked into an integrated app provided you keep logic out of those views! You should be doing that anyway. Also, make sure your tests don't rely exclusively on the views to verify logic that isn't in the view (again, you shouldn't be doing this, but I've seen it a-plenty). That'll keep you flexible enough to transition away should you want to at a later date.
Good luck!