Which is more beneficial prior to PhD: research experience or a second Masters degree?
The question is framed oddly.  A master's degree without research experience will probably kill your chances for PhD admission, at least into the top programs.

Here's a better framing: Is a second master's degree the best way to get more research experience?  Unfortunately, the answer depends on your personal situation.  If you're already doing active research with a strong possibility of formal publication, with well-known faculty collaborators who will write you strong letters of recommendation, then a second MS is probably not necessary.  If that doesn't describe your current research environment, a second MS might be the best way to find such an environment.