The .15 and the .24 has to do with the engine's Displacment size, a .15 is much smaller then a .24. The .24 because of it's larger size will have more torque and have more hp then the smaller sized .15 would. But usually the smaller engine have higher rpm's and can absolutely scream if ported right.

The larger sized engine's are usually used in larger scale rc's and mt's and the smaller sized engine are usually used in smaller scale rc's. But this is not always true, so there are exceptions to this such as the Traxxas Revo and the Traxxas T-Maxx as they both use a .15. But they also have the same model with a .20 in it called the 3.3 but the 3.3 is in CC's and the .15,.24 among others are Cubic Inches. So you have got to watch yourself as some companys may tend to trick people.