I have not used niffy so this is based on their github description, so might be wrong
1) niffy seems to be designed to use UI interactions to lead you to the result, appraise works like fitnesse, separating what is being tested from how it's being tested, so you could go several levels deeper than UI interactions and eg test how your models and views produce outcomes, without the http serving and web pages in between. this makes tests faster and easier to maintain. appraise is more for visual unit testing.
2) niffy seems to be optimised for developers, appraise is intended for cross-functional collaboration between developers, testers and designers. with appraise, the test is a markdown page that renders nicely in github and shows expected results and the conditions under which they should happen.
1) niffy seems to be designed to use UI interactions to lead you to the result, appraise works like fitnesse, separating what is being tested from how it's being tested, so you could go several levels deeper than UI interactions and eg test how your models and views produce outcomes, without the http serving and web pages in between. this makes tests faster and easier to maintain. appraise is more for visual unit testing.
2) niffy seems to be optimised for developers, appraise is intended for cross-functional collaboration between developers, testers and designers. with appraise, the test is a markdown page that renders nicely in github and shows expected results and the conditions under which they should happen.