Method 1 is much more robust and extendable, in my opinion.
Method 1: To add or remove forms, or other functionality, you just add or remove functions. In particular, you can do this from other files, such as separate modules of your plugin or other external plugins. I think this is the main argument in its favor: extensibility and modularity.
Method 2: In order to add or remove forms or other functionality you need to modify an existing function, which is much more bug prone. A switch statement like the one in method 2 easily gets out of hand. The list of cases can get very long, and it is easy to introduce bugs as soon as you have several filters with the same kind of switch statement. For example, you may want filters for validation, display of empty forms to be filled, display of the content of filled out forms, database management,… So now you have a bunch of functions each with a very long list of switch cases, that you have to keep in sync.
(I had some bad experience of this with a popular extension for gravity forms – it’s not unmanageable if you are disciplined, e.g. keep the list of cases in the same order in all functions, but it’s not pretty either.)
Locating bugs: Much easier with Method 1: the culprit will usually be the newly added filter or form, rather than some typo inadvertently introduced in that very long function of Method 2.
Examples: You’ll find tons of examples of Method 1 in the wordpress core (e.g. https://developer.wordpress.org/?s=post+type&post_type[]=wp-parser-hook), but I don’t remember a single instance of Method 2.