I tried both free versions. After three days of evaluating shiva I decided that Unity was the best choice for me. It costs much more, but I have a lot more fun while creating games.
In shiva you have access to most features even in the free evaluation version such as dynamic shadows and render to texture. It is easy to set up and you can create some beautiful scenes pretty easily once you find the restart button. The tools in shiva is generally hard to use and at first glance there are a lot of windows to keep track of. I felt very frustrated after three days of evaluation. I had high hopes beacuse of the price. Maybe I should've given it more time, it is still a good engine, but in the end it was LUA combined with a frustrating GUI that pushed me towards unity.
Just have to say that I'm really impressed by the unity community and all user submitted content on the wiki and forums. Just amazing.