For me, the most important thing is location. A gym is a gym. If you are going to be training multiple days per week, you need to find a gym that is close by. There are only so many hours in a day. If you need to drive multiple hours each day to make it to the gym, you will be more likely to skip workouts.
Second factors I consider - hours (I like 24/7), equipment quality, cleanliness, how busy it is, atmosphere, etc.