I prefer when the same girl is used. But I understand that for a female MC it is very hard to have the same female MC model for all the scenes. For male MC games it's easier if you carefully pick your models.
A couple of advices for real porn game makers :
Don't use a girl who made just 20 scenes if you want to make her an important character. Be aware that probably less than 20% of a girl's scenes will be usable for your game (assuming you want to have your girls in different situations and not just fucking). And yes, it means that you should use popular actresses for your main characters, even if they've been used in other games (we see the same actresses/actors in all our mainstream movies and everybody seems to love that).
There are exceptions, but in general girls working for american studios will have more diverse scenes in different settings. I love euro girls but they are mostly used on gonzo scenes that have a limited interest for a game (for example, many american studios have non-sex scenes that help illustrating the story of the game outside sex scenes, and yes again there are exceptions). Also, with again a few exceptions, girls working in Europe rarely shoot scenes with girls working in the US, so you will have a hard time to find scenes with euro and american girls together.
You can use this
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(preferably when you first pick the girls for your game), among other things it will tell you all the scenes actress X and actress (or actor) Y did together (click on Check Scene Pairings). It will help you if you pick your girls if you plan doing threesomes or lesbian scenes.
