I would like to have an "expense" table and input expenses, but some expenses have multiple categories and should be split like quicken splits expenses into categories and amounts totalling the expended amount. Should I do that with a repeating field (my preference) or generate a separate table?
I recommend the related table. This will make it much easier to generate reports that compute subtotals of your expenses grouped by category. Even with a related table it is possible to create a layout where your expenses are arranged in columns if that's why you want to use a repeating field.