Blender is free so go get it and there are a million tutorials to get your feet wet

You can get a trial of ZBrush 2018: https://pixologic.com/zbrush/trial/ do that also and play with it. Soon you'll get a feel for them and be able to decide. You don't really have to choose because Blender is free :) Also, Blenders' unwrapping algorithm is very good.