Contact-rich manipulation requires world models to reason over complex contact dynamics from multimodal sensory observations. However, it remains unclear which representation properties fundamentally support stable long-horizon planning in contact-rich settings. In this paper, we present ContactWorld, a benchmark and systematic empirical study of vision-tactile world models spanning 12 contact-rich manipulation tasks, including insertion, disassembly, screwing, and exploratory interaction. Across extensive experiments, we find that representations that are both spatially structured and temporally continuous consistently achieve the strongest planning performance. In particular, point-cloud observations improve average planning success rates from 20.7% with wrist-view observations and 22.0% with front-view observations to 32.1%. We further find that the effectiveness of tactile sensing depends critically on cross-modal representation compatibility rather than modality scaling alone. Combining point-cloud observations with tactile force-field representations, which preserve richer spatial structure and interaction dynamics, further improves performance to 36.1%, yielding the strongest overall planning performance across all evaluated tasks. Moreover, tactile sensing becomes increasingly important under long-horizon planning objectives, where compounding prediction errors and contact uncertainty accumulate over time. Together, these findings highlight the importance of representation structure, multimodal compatibility, and long-horizon robustness in vision-tactile world models for contact-rich robotic manipulation.