The Citi Diamond Preferred is a better card with its offer of 0% purchase and transfer intro APR for 21 months.
The PenFed Gold only offers a transfer intro APR of 0% for 12 months.
Although the Citi Diamond wins with its long intro APR period, the PenFed Gold has lower regular APR and 0% foreign transaction fee.
They both have a 3% (min. $5) balance transfer fee and $0 annual fee.
It's really just...