I think that the biggest difference between these two boats is that the M 33' was designed more for coastal cruising, "weekend club racing and over-nighters " whereas the SC line of boats was built to be blue water capable from the get go.
Here is a review of the SC 31' but I imagine that the same build quality went into the 28'. The double-ender design is used for all (or most) of the SC line and is supposed to be helpful in following seas.
Southern Cross 31 Review : Bluewaterboats.org
Interestingly you will not find a review of any Morgan sailboats on the bluewaterboats.org website but builders like Pacific Seacraft are well represented with their smallest, the Nor'sea 27' (also a double ender design) included in the list.
As is often said on this forum, with older boats it becomes more about how well a boat was maintained than the initial build quality. The "blue water" corollary might be that ocean going boats need re-fitting more frequently and should be more heavily rigged than their inshore counterparts. Simply adding heavier rigging to a coastal cruiser may not be enough to make it suitable for ocean cruising if the hull and chain plates can not take the extra strain. Everything about a "blue water" boat is supposed to be overbuilt and designed to be that way which is why I am still hoping that one of the heavyweight design people who are on this forum chime in.
Sounds to me as though you already own the Morgan 33' but are eyeing the SC 28'. If you own both right now then you have been badly bitten by the bug.
None of this is to say that you could not take a M 33' to Hawaii or down to the Panama Canal etc. but if I had to be caught out on the ocean in horrible weather I'd (ideally prefer to be on shore) prefer to be in a boat designed to handle the abuse the ocean can throw at it - and that would be the SC.