If you run a game with the regular crowd, they know what to expect, they play the way you expect them to play, you facilitate the play experience the way you'd normally facilitate the play experience... it a low risk environment, ot's almost a control group wnere you know how most of the variables will play out before the session has even started.
Running a game at a convention has the added wildcard of random players (or at least players who you don't meet up with very often). Often if there are players you know, there might be a couple of subgroups, and there will be an added dynamic temsion between them. This ramps up the chaos, it's probably not as good for testing the fundamental concepts of a game because if things go wrong, you'll never know if it was the underlying system at fault, explanations of the system, uncooperative players, or simply a nexus of negatives. On the other hand, if it works in that kind of situation, then it's so much more satisfying to know that the game can handle these variables.
Years ago, when testing games, I'd love to run them at one of the mid-sized conventions with a couple of hundred players, where I'd often be able to run the same game half a dozen times or more with different ranges of players. That way I could test the same systems under a variety of conditions, and could then account for "player variation" as I analyse the outcome.
Alas, at EttinCon I only run a single session each time, and that's been my main source of external playtesting in recent years. So I need to make sure that single session counts.
6 months ago, when I launched the game, I had two players... so I know it works in that minimalist format, which is great. This time around, we'll have a few more players. But I haven't really tested some of the newer ideas for situation and equipment dice, so those will get a thorough critique over the weekend, along with some of the more esoteric and mystical ideas in the game that will lead back into game designs like Familiar and Walkabout (both of which I'm hoping to get some progress on this year).
Today's work was rebuilding the centrepiece for The Law which seemed to generate a lot of interest for the game last time around.