Why choose when you can use both simultaneously?
The SLAX syntax is signficantly easier to develop scripts, no questions. When Junos processes SLAX it converts it first to XSLT. It is possible, and sometimes necessary, to embed native XSLT into SLAX. For example, if one needed to create a function (not a template) for your automation, you would currently need to use the <func:function> XSLT construct.
Improvements and advances in the SLAX syntax are in progress, and be on the lookout for new features and capabilities.