OK so I’ll preface this by saying I’ve only been paintball-ing a couple of times, compared to Airsoft-ing for years.

So “where do hits hurt more” is the question not “which sport hurts more”, but there is a marked difference between the two. The two sports both fire projectiles at roughly the same average speed ~300 fps (with Airsoft bolt action snipers going much higher up to ~500fps).

Q. So if the speed of the projectile is similar then what is the difference?

A. Projectile weight/mass.

Put a paintball and a BB side-by-side and the difference is evident. A paint ball (~17mm) is roughly 3x times the size of a BB (6mm), but the weight difference is at least a 10x difference –

avg paintball weight: 3.0 grams
avg BB weight 0.2/0.25 grams

So it should be easy to understand that sending a projectile that has 10x the mass (moving at a similar velocity/fps), will have far more kinetic energy and thus hurt a lot more when it hits you. This tallys with my own experience – in that, after a day paintball-ing I was covered in bruises, whereas after Airsoft-ing for the day I have a few welts here and there but most hits were felt but absorbed by clothing.

So with that out of the way… which locations hurt the most?

I find that painful locations are crotch (obviously), insides of arms/legs, neck and anywhere in the ribs. Basically anywhere where you have little/reduced muscle or fat tissue to absorb the impact, is going to hurt a lot.

However three factors affect the pain a lot:

1. Range from the shooter – the further you are the less energy the BB has by the time it hits you.
2. Your amount of clothing – obviously wearing a full plate carrier and thick BDU military style clothing helps absorb BB impacts.
3. The weight of the BB – a heavier projectile will retain/carry more energy and just feel like a harder hit, even if fired at the same FPS.

Thanks for the request, and I hope that this helps!

UPDATE

So having read some of the other answers to this question, whilst most agree that paintballs hurt more there are other talking about the area of the projectile when it hits you and the energy transferred.

So using the numbers I already have, I went to (ENERGY CALCULATOR) to calculate how much energy each projectile has straight-out-of the barrel.

6mm 0.2g BB = 0.84 Joules at 300FPS

17mm 3g paintball = 12.5 Joules at 300FPS

Quite a difference! To address the issue of the paintball have a larger surface area whereas the BB does not compress at all I consider the largest cross sectional area of each projectile.

BB:

radius = 3mm
flat face area = 28.26mm^2
Joules = 0.84

=> 0.84 / 28.26 = 0.0297 Joules per mm^2

Paintball:

radius = 8.5mm
flat face area = 226.87mm^2
Joules = 12.5

=> 12.5 / 226.87 = 0.0551 Joules per mm^2

So the paintball has nearly double the energy per surface area of the BB!!

Even if you were to DOUBLE the surface area of the paintball (to allow for some crazy exaggerated deformation) then the energy only drops to just below what the BB has.

Paintball (deformed):

radius = 8.5mm
flat face area = 453.73mm^2 (area doubled to simulate exaggerated deformation)
Joules = 12.5

=> 12.5 / 453.73 = 0.0275 Joules per mm^2

So it’s easy to draw the conclusion from this that the paintball is carrying and applying more energy when it strikes you than a BB, and even if it deformed to twice it’s size it is still almost the same. Relate this to real world evidence and it would appear that more energy, even spread over a larger area, equates to worse soft tissue damage than a BB, hence the worse bruising from paintballs, compared to little red marks from BBs.