Average-level starting characters or complete beginners (like in Cerberia) – which is better?
First, due to my personal time constraints, I probably won't be participating in an experience-based game. No matter how interesting it is, or how many emotions it evokes, I don't have the willpower of Fragment/Artz/Heroist to log in once a day or once a week (and this is without any irony; they are great at prioritizing and still manage to come online and write insightful posts that deserve to be framed). Other players have varying levels of involvement, but I personally feel like I'm mentally engaged in an experience-based game almost 24/7, which is not healthy in any way. So, I apologize, but I'm unlikely to continue.
Second, based on my own experience, and knowing that I'll still be reading your posts 100%, I've always been more interested in beginners with a significant advantage. As in Cerberia, and even more so in RI3, it was great to nurture a character from scratch and then watch them grow and become capable of more, earning someone's respect (or, conversely, unexpectedly gaining powerful personal enemies 😵). Based on the same principle, I initially didn't want to "isekai" anyone in AiA, and creating Aiko proved that this game was also well-suited for a low-level start.
Now, about average-level characters. Although this concept isn't inherently bad in a vacuum, and sometimes it even works (in Nebka's last experience-based game, our group really felt like a solid mid-tier team, already with an established reputation), in reality, it creates a lot of problems. Ideally, you already have a background, connections (if you're not an "isekai" character), a certain professional pride (you probably wouldn't go on a quest against rats, unless it's your personal quirk to complete all sorts of minor tasks), and, yes, this should be noticeable in terms of gameplay. As a result, all of this leads to greater challenges for the GM in creating that "pro" atmosphere, and limitations on possible quests and storylines... Even to higher expectations, because you automatically want certain organizations and individuals to recognize you, sometimes even to the point of friendship, if there are reasons for it in the backstory. At the same time, as a player, you are an absolute beginner in the setting and can easily make a mistake because you don't know something important.
I am convinced that it is no coincidence that even in video games, authors almost never manage to create a start with a "mid-level" hero: even if the prologue tells you that you are incredibly cool, in reality, you start at level one, with a rusty sword, and hunt goblins (and it's good if it's not jerboas).
Therefore, yes. Beginners are better than average-level characters, if you look at it realistically. If you want to challenge established patterns, I would be interested in seeing a good implementation of experienced soldiers, but I wouldn't say that I'm particularly excited about this possibility.
Ultimately, the GM always acts according to their vision, because the game's success depends on their enthusiasm.