I normally will choose the same brand as the servers I using. E.g. HP servers use HP racks. IBM servers use IBM racks. Reason is that the server will come with a mounting kit that best fits it's own brand of rack.
Of course most 13 inch servers will fit most 13 inch racks. U can install the nuts to mount servers to any racks. But based on experience, getting the same brand makes it easier to mount them
Power gangs also have specific holes to mount them onto their own brand of racks. E.g. IBM power gangs u can mount them on the inside of the side panel using the holes specially situated for them. Of course, most power gangs also come with generic mounting kits. I just find using the genetic mounting kits makes mounting it less clean. Plus, some holes might be blocked, making it a hassle to mount new equipment in the future
That being said, I like IBM racks. I find the door is more durable than HP ones. The grill on HP rack door is quite flexible. Accidentally kick it and u get a dent on the door