I like Harald better. Rocks had moments where he was fun, but I didn´t particularly like how Oda handled him, imo it was wasted potential. Firstly, it just sticks out too much that his design looked way too much like Blackbeard. Secondly, I feel like we were told too much about him, as opposed to actually seeing his adventures. We were told he broke the gates, broke international law,killed the admiral, etc, but Oda really didn´t show much of substance, nor did we really get an explanation for his motives, or why he never built a good relationship with his crew. Oda did bait us with the stuff about Davy Jones, but I don´t really like that. It feels like Oda is constantly trying to build up new characters, but in doing so he undermines the current ones. Rocks´s existence was that in comparison to Kaido and BM, and the same happened to Rocks with his ancestor in Davy Jones.
Also, I disliked both of them losing their agency as a plot point, but at least Harald made his choices that led to that. Rocks sort of was randomly selected among the GV 6 to get DR´ed, and the timing is kind of random too, since Imu could have done that to him any time during the 17 years since he broke into Mariejois. It´s hard to take anything away from that. Haralds death in the castle was imo, a bit more well written of a conclusion.