When we talk about table top thickness, it’s important to differentiate between a 2D and 3D table top. If you’re simply taking a flat plate and welding some legs on it, a thicker top will result in more torsional rigidity, as well as resistance to deflection. However, if we’re talking about a modern 3D fixture table, these elements are already covered by the engineered design of the table with its supporting ribs and sidewalls.
A 3D table is about 30 times stronger than a 2D table, so then the real consideration with the surface plate thickness comes down to durability.
For the vast majority of users, even a 1/4” thick plate top, supported by dozens of ribs, is going to last a lifetime. If you’re planning on building bulldozers on the table, and will likely be dropping heavy components and abusing it like crazy, a thicker plate will provide more resistance to impacts and abuse.