MS-Contin has problems that are not found in Oxy specifically "first pass bioavailability" in which as much as 60% of the morphine sulfate is destroyed by your liver. So if you take a 30mg pill, your body will actually get only about 10mg worth of active the ingredient.
Also, the onset time is between 1 and 1 1/2 hours. This is a long, long time to wait for pain relief. Even if you "break the rules" and chew them, there is still a long delay before the morphine is absorbed in the intestines.
And, like Oxy, MS loses its effectiveness rather quickly. I know of one patient that went from 15mg to 90mg (daily) just because he developed a normal tolerance as time went by. This creates other problems, such as constipation, that require stool softeners and laxitives before long to prevent blockage.
The good news is that MS-Contin is much less controversial and slightly easier to get perscribed for chronic pain. And if used IV it is very, very effective.