Both aren't easy for beginners. Fallout 2 has the easier script language. FOnline uses AngelScript, which is a bit like C++ or something. Fallout 2 has more limitations such as map size, amount of colors in graphics, etc. while FOnline has not. In Fo2 you do everything in script (dialog, interactions, etc.) while FOnline has a separate dialog editor with a tree-view.
Me personally, I'd stick to Fo2, as the mapper is less of a clusterfuck and we have a pretty good script editor by now. Also you can use any original script as a reference for something you'd like to do. Thanks to Sfall, there are also a lot less limitations than 10 years ago.