This is going to depend on two factors. What was your character’s training in? And, who has your untrained fighter been going up against?
If your trained combatant has been taking physical fitness classes, that won’t translate smoothly into combat. If they’ve been training in self defense or for practical combat, then their lack of “live” experience is still a significant factor.
When you’re training in martial arts, you’ll spend a significant amount of time practicing your techniques on your fellow students in a safe environment. The point of this is so that you can translate these techniques over into the real world if you need to.
In this sense, a martial artist who has spent years training in a practical martial art could easily be more “experienced” than a brawler who gets into fights with college kids behind a local bar every weekend, but end up in trouble if the experience doesn’t translate smoothly. There are plenty of recreational martial artists who get caught or get into trouble if their training hasn’t spent any time preparing or simulating a real street fight.
Military training is generally better about this, and the trainees train with the expectation that they will go out and use these skills in the real world. In this case, training is heavily dependent on who is doing the training and what they are training for. Is the character who as been trained but experienced a real fight been taught to apply their knowledge? How quickly do they learn on the fly? What is their reaction when under stress? Have they been in stressful situations before when forced to make quick life or death decisions?
It’s not just training versus no training but experience. The people in question come into play, the level of training comes into play, and the intent behind that training comes into play. A character can come into a fight with no experience but better preparation, better gear, a stronger mindset, and fresher than their compatriot who had to learn their lessons the hard way. They could fight by rote or they could be inventive, worked to understand current battlefield techniques, and sought to overcome them.
The kind of person in play on both sides is just as important if not more so than their backgrounds.
Generally speaking, self taught fighters don’t know what they’re doing, and don’t really have the fundamental training to understand what they’re seeing. They can mimic the overt actions they see, but not the gestalt of a technique. For very basic techniques, that’s enough. But more subtle techniques can completely elude, and neutralize, them.
The cliche of a fighter who copies other martial artists isn’t a complete fabrication. But, doing that first requires an extensive and varied formal education in Martial Arts. It isn’t something an untrained fighter can just pick up on a whim. They can work to become better than they are, but they need to realize that what they have isn’t enough to begin with. There’s ultimately a ceiling that they will hit where they can’t continue to improve on their own, even with all their experience. They’ll plateau out, often earlier than the inexperienced fighter who comes in with better groundwork and a better base to work from.
It’s worth pointing out, martial arts training can occur anywhere, and the quality depends on the instructor’s ability to teach and their student’s willingness to learn. It really does not matter if they have a formal dojo to work with. So it is possible to have a street fighter who has very good training, and is quite skilled, without ever having set foot in a traditional training environment. But, it’s critical to remember, this is not someone who taught themselves.
It isn’t as simple as rolling the dice for X versus Y, both can be very dangerous.