Haha, when I read the title of this thread I was thinking of posting (jokingly): 'Do Tax law or Company Law'.
Company Law is slightly easier than Tax law in my opinion but still is not the easiest module that I've studied (to be honest, I'm not completely sure which is the easiest I've studied). I say Company law is slightly easier because its easy to pick out all the key cases and additional areas of law that you really need to get to grips with before the exam. As long as you focus on them and really understand them, the exam shouldn't be too much of a problem.
On the issue of deciding which modules are easy. I'd have a look at they syllabus and judge how broad and detailed it is. If it seems like something you can't handle, then that might be a module to put on the side. Also consider how the module is assessed. Is it 100% exam based or is part of it assessed through other ways, coursework etc.
Some modules might have a more theoretical angle; like modules titled 'The Lawyer & Society' or 'The Law and Society' etc. Also, talk to students who have recently gone through that particular year at university.