Joffrey was rebellious, independent, willful and assertive, and found ways to escape his mother's control. Plus he was older. I was thinking Joffrey was more like 13 (could be wrong I guess), but even if he was 12, that's still much older than 8, a significant difference. Tommen is both much younger (so he doesn't know much about being king and has little interest in it) and much less willful and assertive. Cersei seems to have done a better job of breaking any sign of rebelliousness in Tommen than she was in Joffrey. Though even with him, there are already some signs of trying to come into his own and weaken Cersei's hold over him. He's not there yet, but if he lives longer than he did in the show, he might yet become a ruler in his own right.