There is an Esri blog post that describes the technique for doing this in detail called If you are stuck at "if" – Part 1:
Part 1 – Gives examples of quick and dirty ways of using the Calculate
Value tool to create branches using if statements in a model.
In your case you want to check for the existence of a feature class rather than a product license so I would use this as the code for your
Feature Class exists Python script tool:
and this as the code for your
Feature Class does not exist Python script tool:
Personally, I far prefer to implement such branching using pure Python/ArcPy rather than having ModelBuilder in the mix.