Hi,
I think the basic difference would be that SELECT ..FOR UPDATE would be when you want to select to update the entry and do not want any other modification in between. The Row is locked after the SELECT is executed. a scenario where the expectations is to lock the entry and not just do a READ LOCK or WRITE LOCK as can be achieved by the LOCK OBJECT.
Also the SELECT...FOR UPDATE behaviour depends on DB system as to what it may lock and how such update is handled. Where as in LOCK OBJECTS the way the Locking is handled in more explicit in nature and is in control of the programmer. The links should help:
http://help.sap.com/abapdocu_70/en/ABENDB_LOCK.htm
http://help.sap.com/saphelp_nw04s/helpdata/en/cf/21eea5446011d189700000e8322d00/content.htm
Cheers,
Arindam