Both the Victoria service and the one that leaves after it, are both popular. For those going to Merstham/Coulsdon/Croydon, the Victoria service is the most attractive, as it leaves before and won't be overtaken. If you want Thameslink core, the answer is obvious.
Most Reigate originating services become full at Redhill with some seats left, and standing room normally around Purley/East Croydon. It is a popular service and the Thameslink trains do have no seats by East Croydon on Redhill Line services.