Pebble tec is made with cement and pool mix, but the pool mix is made with silica or quartz instead of crushed marble. Silicate is a far stronger and more durable aggregate than marble is. When the surface is finished with Pebble tec, the surface is mostly made up of silicate.Pebble tec will stand up alot better to acid. For example, if you mix Muriatic acid with marble, then the marble reacts and degrades. Pebble tec will not degrade as quickly, which means it can stand up to the elements and cleaning agents better than plaster.
Related: How to finish a concrete surface
Plaster for something such as a pool is made of cement and pool mix, which is crushed marble. When a surface is plastered in this fashion, it means the surface is mostly made up of crushed marble. If you neglect your water-chemistry, then a marble finish may become damaged a lot easier than a pebble tec finish. This is also true if cleaning chemicals are misused when cleaning the pool or the pool finish.
Pebble Tec vs Plaster
Here is a quick table of pebble tec vs plaster showing the differences between a pebble tec and plaster finish.