A belief in "spirituality" assumes the existence of something outside ourselves into which we can tap (or that can communicate with us in some way), while sentimentalism is nothing more than the manipulation of emotions that are common to all - except psychopaths who are the exception that prove the rule. Another term that is used interchangeably by many for sentimentalism is emotionalism. The difficulty in religion is that the latter (emotionalism) can be used (intentionally OR unintentionally, with good OR evil intentions, toward positive OR negative goals, with wonderful OR terrible results, etc.) to approximate the former (spirituality). It happens in the LDS Church, but not one bit more than in any other religion - and far less than many.
Perhaps the best example is the idea among lots of people that tears are a result of an encounter with the Spirit - or, to be more precise, that if you cry in church it is because you have "felt the Spirit". I'm not rejecting that as a real manifestation of real spiritual experiences for many people; I'm saying it's easy for someone who knows how to do so to cause tears, as evidenced by Hollywood and the music industry. For me, the distinction between spiritualism and sentimentalism in those cases when tears are flowing is less about the tears and more about the cause of the tears - and the lesson learned through the experience.
I don't like to draw a bright line between the two and label one as bad and the other as good, since I don't think it's easy (or even possible for lots of people, including myself) to understand perfectly what is a result of exposure to an outside "spirit" and what is completely internal. I also am not convinced that something that is entirely internal must be sentimental and not spiritual. Personally, I think nearly all of the grand visions of our scriptural canon might have been completely internal (visions, not visitations) - but I am totally fine with them being spiritual and not merely sentimental. I say that mostly because of my own experiences.
I've had a few experiences that I personally believe to have been "pure spiritualism" generated by a contact with something outside myself (not just natural reactions totally from within or caused by other people), but, by and large, the vast majority of my spiritual experiences really could be analyzed by someone else and deemed to be nothing more than my natural emotions being magnified or manipulated in some way that is not "spiritual". I believe there is a "Spirit of God" (whatever that means), and I believe there is a God (whom I describe as a Father), so I choose to believe that I have had spiritual experiences - even as I recognize that some of them might have been nothing more than emotional reactions to powerful stimuli. In fact, one of the most powerful "spiritual" experiences of my life came about through a series of events that I know many others (including some here) would see as nothing more than contrived manipulation (NOT by the Church) - but I believe it was spiritual not sentimental more because of what it taught me (the lesson I took away from it) than because of the exact nature of the experience.