In general, I do find that Diplomatic strategies in negotiation decks far more effective than Hostile strategies. I will add the disclaimer that I'm not sure if Hostile cards are just farther behind in the design process. Here are my main reasons for consistently going Diplomatic:
-Flatter (from upgraded Sal's Instincts), is a huge payoff for having Influence, especially now that Overbear has been nerfed.
-Big damage through Good Impression, Beguile, or Intrigue.
-Steady Composure gain. With recovery and healing being at a premium, especially at higher Prestige levels, a strategy that minimizes the Resolve damage you take quickly pulls ahead. Less Resolve damage taken means less money spent buying drinks to heal Resolve. A more resilient negotiation deck means you can ask for more money on every job, knowing that your Composure will minimize the risk...if you don't just quickly obliterate the extra money arguments with Flatter. The money you earn then snowballs to make the deck (and your battle deck) even better. Cards like Airtight or the upgrade to Fast Talk that adds 3 Composure can keep an Influence-based deck from taking any lasting damage to the Core Argument. (Being able to last longer and draw out a battle can also allow more upgrades, but the experimental build added fatigue to combat this, which is a feel-bad solution but probably a step in the right direction.)
-Card advantage. I made a deck with two upgraded copies of...Swift Retort, I think? It's the card that improvises two 0-cost cards from the discard pile. It was often going infinite by recycling Pale upgrades to Ergo (0-cost, Improvise a card from the discard pile) and Second Wind (0-cost, gain 1 action per 3 cards in the discard pile). Even without going infinite, repeatedly reusing Solid Points for damage/Influence or 0-cost upgraded Good Intentions was highly effective. Magnetic Charm and Visionary Setup could cycle through the deck quickly to find the right pieces. I don't object to engine decks—it was a lot of fun—but once I had it built, I was skipping virtually any offer of a new card, which I likely would have continued doing even if the later chapters were in.
-And, finally, the secret reason Diplomacy is the way to go is actually NOT a Diplomacy card. It's Scorched Earth. This card is very often an instant finisher if your deck doesn't have any other Hostile cards, even before being upgraded to 3 damage per Expended card. I would strongly recommend a change to this card, perhaps only having it Expend Manipulate cards, which would both tone down its power and make it equally effective in Hostile decks.
This is my personal experience up through Prestige 6. If Hostile is viable and powerful too, then that's great. I just haven't seen it take off in the same way.