Ironically, Diplomacy adds more to the single player game than it does the online multiplayer game. Many online Diplomacy games feel just like Entrenchment games, especially ones where no one starts in an eco slot, such as 2v2's adn 3v3's. 1v1's in Diplomacy are no different than in Entrenchment.
I think you will find that online multiplayer is much more challenging but also more interesting and fulfilling than single player. Human opponents are capable of real strategy, turning the game into a space chess match, wherease the AI is retarded in these regards. No matter how much of an economy-boost the AI receives, it's still just a dumb brute and not a real opponent.
If you want to play it online, it might be better to start out playing the original Sins online first for three reasons. (1.) You're more likely to find new players on regular Sins. (2.) You don't have to worry about learning how to deal with and manage starbases (against human opponents). (3.) The focus of regular Sins will be fleet-to-fleet battles, which is still the meat of Entrenchment and Diplomacy anyway. To become a good player, you need to become proficient at fleet-to-fleet combat.
That having been said, many of the online multiplayer games are played on Diplomacy and there are many new people on Diplomacy, too, but at least half of the players are experienced. Do note that the larger games, the 4v4's and 5v5's, are on Diplomacy now. For regular Sins and Entrenchment you're more likely to find 1v1's, 2v2's, and sometimes 3v3's.
I think your best bet is to start out by playing Sins and Entrenchment online and to see if you like it. If you do, then consider getting Diplomacy.
