There's no need to compare really. There are differences of course, but the primary differences between the two preparations are pretty obvious and already known to you. Pills were "invented" if you will as a means of easily and accurately dispensing medications to folks. In medicine, oral solutions are used specifically for specific cases in which there is a direct advantage.
I had my jaw wired shut one time. I received Morphine syrup for analgesia. Patients who have a hard time swallowing, think hospice, often benefit from high-potency oral solutions, as keeping anything down toward the end of life can be difficult.
There's no need to overcomplicate things. Your Morphine could be a pill, a solution, a capsule whatever. It's still Morphine and you should treat it just as such.