Depends on what types of internship sites/jobs you'll be looking for. I was a RA for a couple of years then a TA. A RA definitely is better for research & publication opportunities, but I don't regret teaching, either. It's advantageous to have some experience in teaching just in case I'm ever interested in working at places where teaching is emphasized (like small liberal arts colleges). It's good to show you have some experience teaching lecture courses and also have a portfolio (e.g. your syllabi, students' feedback, etc.) to show for it. If you're applying for more research-heavy internship sites, though, the RA would be better. I don't think I found a site yet where my teaching experience was really -that- helpful in making me more attractive. Some people commented on it in terms of how it may signify my capabilities to run community workshops, but nowhere as helpful as my RA experience was in interviews with more research-heavy internship sites.