Mercenary: "use this power to gain defender rewards as though you were a defensive ally ".

Goul:"use this power...to gain defender rewards for each ship you send to the warp"

CI Rules: In this variant, defensive allies may draw from a special,slightly more powerful deck of cards when receiving defender
rewards . This variant makes use of the reward deck.

SO

Mercenary attacks with 4 ships and wins, sending 8 ships to the warp. It can receive 4 cards from REWARDS DECK or 4 ships and/or 4 card from Encounter Deck.

Goul attacks with 4 ships and wins, sending 8 ships to the warp. It can receive 8 ships and/or 8 cards from Encounter Deck, but nothing from Rewards deck as its not counted as a defensive ally.

Correct?

There are two types of defenders: main and ally. There is no defender reward for the main player (other than keeping the colony), so I'd say the defender reward for the Ghoul would be the one for an ally - the Ghoul can draw from the special reward deck (but that deck might not be in the game, since it's a variant).

Another important difference, pointed out in the corresponding thread on BGG, is that Mercenary can be used as a main player or offensive ally and Ghoul can only be used as a main player. So, although the Ghoul's is more powerful, the Mercenary's can be used more often.

Yeah, you are dead on in that point. I was originally thinking only the Merc could draw from the Rewards Deck but after as The Warp about it, its that both can draw from the Rewards Deck.