|The lighting setup used for the comparison shots.|
Although we've seen each of the three images and talked a little bit about the quality of light for each one, it's not really until you put the three images together that you really get to see the different qualities of the off-camera flash sources...
|Nikon SB24 speedlight|
|Interfit Stellar Extreme|
You can see from these comparison shots that there are shifts in colour for each of the light sources, but perhaps the most obvious difference is between the Profoto and the Nikon flashes; that Profoto light is just so much softer (relatively speaking) than the Nikon. The Interfit unit definitely sits between the two others.
Which one is 'best'? That depends on what you want to achieve, what your budget is, how portable you need to be, and what light modifiers you might want to use. The thing to really take away from all this is that it's worth experimenting with your lights - whether they are continuous lights, speedlights or studio flash heads - and learn about the quality of light you get from using them off-camera.