Both jetting and snaking clear blockages. Only one actually cleans the pipe.
Snaking (mechanical cabling)
A snake bores a hole through the blockage. Water starts flowing. You go home. In three months the grease and root growth fills the pipe back in and you're calling for another service call. Snaking is fast and cheap, and for one-off clogs from a foreign object, it's often the right tool.
Jetting (high-pressure water)
Jetting scours the entire inside diameter of the pipe at up to 4,000 PSI. Grease comes off the walls. Roots get cut. Scale breaks down. When you're done, the pipe is genuinely clean, not just passable. On lines with recurring blockages, jetting buys you years of clean flow.
When to snake, when to jet
Foreign object stuck in the line (kids' toy, mop head, etc.): snake it. Recurring grease, root intrusion, sludge, or scale buildup: jet it. First-time backup on a commercial or grease line: jet it - you're going to end up jetting anyway once it comes back.