How about you make 2 different saves and do both? I did.
Personally, it has no real impact either way. I chose to continue the game after killing Playboy X.
Kill Playboy X and you get his pad (always useful to have a safehouse in the top part of the island), a secret outfit (Claude's from GTA 3) and if you befriend Dwayne enough you can call on a pair of his boys to go into battle with you. Useful cannon fodder as the AI's a bit crap.
Useful yes, important to the game? Not really, plus if you befriend Dwayne he's ALWAYS MOANING like a whiny emo bitch - I wished I capped him after all.
Kill Dwayne and you get some cash but that's it.