I use both, but it will really depend on the hair. With any natural material, there will be some differences from one animal to the next. I've seen belly & body hair that was so similar in texture, it would be difficult to say for sure what part of the hide they actually came from. For bass bug purposes, where you're making the majority of the body with spun & trimmed hair, I look for hair that has a thick, course fiber. Fine hair will work too, but I don't feel it packs as well, nor floats as well.
I've also processed & dyed a lot of my own materials, and the white portion from the belly, or the rump just below the tail, is usually best for really bright, vibrant colors. For darker colors, or where I desire a mottled affect, then the darker body hair is what I'll use. Sometimes the darker hair from the base of the tail is quite suitable for spinning, especially after it's been dyed.
You will see hair advertised as "Northern" which generally means the hair came from a deer harvested in a northern state. But, this does not always mean the hair will be any different than that which is taken from a deer harvested in a southern location. Again, it really all depends on the animal it came from.
Yes, generally that which comes from northern deer, may have hair that is thicker and denser due to more extreme cold temperatures in northern states.
But, when it was harvested, as in fall versus late winter, and it's age may also make a difference. Still depends on the individual animal.
Unfortunately, when buying packaged hair, and especially from online sources, you get what they have in stock, so it may not always be the best for this purpose regardless of what they call it, or what part of the hide it came from.
The best source I've found for purchasing this type of hair is from Chris Helm who owns & operates Whitetail Fly Tying Supplies in Ohio. Chris processes his own materials & knows which type of hair is best for specific tying purposes. He'll even talk with you about what you need if you give him a call!