I once made an analysis about the three types of dragon slayers a long time ago. I consider the 2nd generation DS with lachrima 'safe bets', meaning they get automatically a large, stable source of power (what I mean with stable is a direct source of power that the user has without any kind of physical or emotional boost) and have to do very little for it.
3rd generation DS then are the same, only their stable power source has increased to a point they can even go into a controlled Dragon Force mode. The downside of both however is that that lachrima, who is the source of their stable power, at the same time limits their power so that it would not go out of control. It works both as a booster and a limiter. With other words the DF mode of Sting and Rogue are weaker than the ones Natsu has shown so far.
1st generation then are 'risky bets'. Their power is more unstable, it fluctuates more and requires more training to sustain it, but in return they have the highest potential and their max power is hard to measure. Natsu's DF mode was far more superior to Sting and Rogue's as it didn't had any limiter, I doubt however that Natsu can just toggle it on and off like the other two. Remember that DS magic is fueled by emotions, while the other 2 generations tap mostly out of lachrima, the 1st generation taps out of their emotions and even then it's very fickle as Natsu had to eat the Flame of Rebuke and Etherion matter to unleash it. When those emotions reach a boiling point, they probably enable the DF mode instinctively and automatically.
So it's choosing between betting on a champion horse, but getting little money in return or betting on a dark horse, much more risky but you get much more in return if it wins. Quantitively seen is Natsu's DF better than those of Sting and Rogue, however the latter two can activate easier.
Well that's just how I see it.