Hi @abdullah and welcome to the forum.
Unless you need any new facilities that the preview version has added I would start with the stable version.
[To find what has been added since the stable version visit Babylon.js/what's new.md at master · BabylonJS/Babylon.js · GitHub]
To keep the total file size of your project low then use the version babylon.js which has been minified. This gives exactly the same facilities as babylon.max.js which is the readable form. When you are beginning this will probably give you all the facilities you need. However BabylonJS offers more such as an inspector, loaders, GUI etc. If you need any of these you can load them as well as babylon.js.
To see if you need them you can search for them in the docs, so for example putting loaders in the docs search bar leads to how to: Load from any file type - glTF, OBJ, STL, etc.