What to do with old 401k?

Looking for advice for what to do with a 401k from previous employer. I recently switched companies and wondering what to do with my 401k that is with Vanguard. My new employers 401k is through Fidelity. I already have a Rollover IRA and a Roth IRA with Fidelity. Should I move it to my rollover IRA with Fidelity or my new employers 401k with Fidelity. I like the investment flexibility of the rollover IRA, but is there a con to it that I am missing as opposed to rolling it over to the new 401k? Thanks for any insight!