The energy level and sheer randomness of both can confuse outsiders, especially if they're well-adjusted and balanced, but there's a fundamentally ruthless/pragmatic streak in ENTPs that ENFPs lack. The Thinker side of them cares more about ideas than relationships and thus can muster a much more "testing"/sarcastic edge than the more open and friendly ENFP wants. This doesn't mean that ENFPs can't be mean, because they can cut deep when pushed, but fundamentally it's a reaction mechanism.
"The subject chooses to sit in shadow and search for wisdom by reflecting upon his trial. The problem is not that he is cold and wet, but that cold and wet seems problematic, so he embraces those hardships in order to best them."
ENTPs = "the world would be a very boring place without us MUAHAHAHAH *wink*"
ENFPs = "We are the force of rainbow, unicorns, flowers, sunshine in the universe here to purge the darkness!!!"
Here's a more charitable summary on thinking v. feeling
Logical - Thinkers base their decisions on rational thought and logic, and they tend to be objective. They like justice and standards. Thinkers tend to be more interested in ideas, and the reasoning behind decisions or concepts. For them, emotions are secondary, or passing, and of less importance. To Feelers, they may seem cold-hearted and remote.
Personal - Feelers' decisions are often based on "gut feelings." Their outlook is more subjective. They like harmony and often energized though appreciation and encouragement. For Feelers, emotion is more important than reasoning, rationality, or logical discourse. To Thinkers, they may seem soft-hearted and illogical.
Emotional Responses -
Thinkers are as likely as Feelers to experience emotions. However, they are less likely to show those emotions; in fact, emotional reactions can be embarrassing to them.
Feelers are more likely to wear their hearts on their sleeves. However, if this preference is combined with introversion they will not share their feelings openly.
and more specifically on the ENFP/ENTP divide:
Function theory sees the ENFP as much like the ENTP, while intelligence theory sees the ENFP as an Advocate Idealist, the ENTP as an Engineer Rational, the two, again, very different in their behavior and corresponding attitudes. Admittedly ENFPs and ENTPs can be hard to tell apart, at least on short acquaintance, but watch for any length of time and their differences show up one by one until it is abundantly clear that the resemblance is at best superficial.
hoarding time and space
A single event can awaken within us a stranger totally unknown to us. To live is to be slowly born.
— Antoine de Saint-Exupery