If you plan to do more investing, I'd go with a HELOC.

Plan was to transfer the balance onto my HELOC (AMEX will charge a 3% transfer fee) that I just used a portion of to buy two rentals, then pay off completely after the intro period. 

We have a HELOC on our primary residence. 

Would we simply ask for a mortgage or would a HELOC be better?

You can leverage the equity in other ways (HELOC, Refi). 

This can be through a HELOC, refinancing, or if one of your properties are paid off then you can put it up as collateral. 

Is it better to Cashout Refi or Refi and get a HELOC?

This would also give you more liquidity that if you needed additional cash in the future you could maybe get a HELOC or refinance them if needed

or wait for HELOC be offered again?

I've also read that the HELOC products were some of the first loans called during the last recession.