pretty random resemblence if you ask me ;)
You get same feeling when you look in following order on following ships : Sulaco (Aliens) -> Omega class (Babylon 5) -> Halcyon class (Pillar of Autumn / Halo) all those ships bear some resemblence to eachother but I guess the inspiration was pretty loose while designing (overall shape was similiar and that's it)
Defiant is completely different shape and type of design, it's aerodynamic while X303 is bulky and resembles aircraft carriers
did I just spoke like a nerdy smartass ???