The 5" will produce better ballistics, but not significant IMHO to warrant a purchase. That is to say that the 4" and 5" are both capable of getting the job done with good self defence ammo.
I'd focus on what's better for carry. For you. that's what counts.
To me, there's little difference and I carry both a 4" Service and full size Tactical as well as 1911's. When I carry my 4" Service with a SureFire x300, there's about a 1" gap where the light is longer than the barrel. So in this case, I'd rather carry the 5".
So my advice is to get the one you'll carry most often and then practice as much as you can.
I use Blade-Tech. The Tactical Light Holster w/Sting Ray Loop. You can see more on it here: Tactical Light Holster w/Sting Ray Loop :: Holsters w/Tactical Light :: Holsters :: Blade-Tech IndustriesJust out of curiosity, what holster do you use with your 4" w/ Surefire X300?