the trinity is the OLD vanilla, entrenchment and diplomacy expansions, AND after that the rebellion came out as a stand alone 'expansion' that does not need the older expansions and has some DLC available for it. to my very limited knowledge ther is no single package of the complete sins trinity & rebellion & rebellion DLC's.
what to buy depends upon what you want to do, if it is play with friends, then find out which expansion they have/use and get that, if it is to play particular mods, then find out which expansion the mod is for and get that
harpo the ghost NON-subscriber
