I don't see this answered any where on the web, amazingly.
Unit tests in lime seem so much more simple to write than PHPUnit with all its extra boilerplate code.
Are there any definitive reasons (besides "PHPUnit is the defacto standard") for choosing one over the other?
PHPUnit is more Object oriented than lime, I mean in the way you write your unit tests code. PHPUnit has more functionalities ( think Selenium integration and xUnit output standards ).
Lime seems more simple with simple test cases, but when you tests are growing, it's unreadable.
I've used both, and I have no regret with the switch to Phpunit.
Moreover, for technical reasons, you should take care of the future of lime.
For info, Lime2 has been written by Bernhard Schussek, but to my mind, dev has been stopped since Symfony2 is using phpunit.