Go 3.9, you can have the one sitting in my parents driveway for FREE! Come pick it up! Just needs a little work but if you're rebuilding it you're all set.
OK are you talking about the difference between a carbed 3.5 and a Fuel injected 3.9? I've driven both, and there is a good difference between the 2!
3.5 carb ~100 HP
3.5 EFI ~150 HP
3.9 EFI ~187 HP
Those are the starting #s but, short of forced induction there is no substitute for displacement!
Don't forget to Search first then ask questions later! The loose nut behind the wheel