Where are you from, since this can influence your decision?
I can agree that Warmbaths is sometimes overcrowded but so can Badplaas, although, depending on the distance to travel, I would recommend Badplaas. It is a wonderful area and has a holiday and /or queit vibe, depending on what you need. Take into consideration toll fees payable to Warmbaths and Middelburg. Also take season (temperature) into consideration, it will be warmer in Warmbaths now, although the days at Badplaas is nice. I also know that to travel to Sondela you need to drive on a gravel road.
Hope you can make a decision.