As a master's student, will a GRA or GTA position provide a greater benefit for PhD applications?
If you're planning to go into Academia, you might want to do a GTA (Graduate Teaching Assistantship) to get teaching experience. Because a GTA means approximately 20 hours per week on teaching duties, you may find it harder to work on your thesis or dissertation as much as you want.
A GRA (Graduate Research Assistantship) will focus on research instead but you may not be working directly on your thesis research but instead on another project for which a professor has funding.
At my university (25 years ago), a GTA paid less than a GRA so that's also something to check.