I voted for 3 things:
1) Who is running the game
2) Difficulty Level
3) Variants
I think knowing the organizer of the game is important as it gives you a level of trust and an idea of the "ethos" in which the game is played. The worst thing that can happen in a SG is if it fizzles out because people lose interest or it is not organized properly, so having someone who will keep things moving is important. Similarly if the organizer is someone you have played with before or whose games you have read, you get an understanding of the spirit in which the game will be played and whether or not that is to your liking. This overlaps with a couple of other options, teammates and rulesets. So in my personal experience I find myself playing SGs with roughly the same set of people...for example, I have played in loads of SGs with Aggie, Meli and Greebley.
Difficulty level and variants is a personal preference: a game has to be challenging and interesting. If I want to play a simple, non-variant game I tend to play solo. If I want to try a very difficult game or one with an interesting twist then I will either organize a SG or join a variant I am interested in.