I owned the HD280 Pro for almost 3 years, and had the RP-21s for a week (sold them after buying the AKG K601). The RP-21 is better.
The HD280 Pro is a great can, but only after they've been thoroughly broken in and burned in. For the first few months, I struggled with their tight grip, their anemic bass, and strident highs. My well-worn pair is now wonderfully squishy and neutral-sounding, with impressively low (but still anemic) bass - but it was quite difficult getting to this point. The headband has also cracked quite a bit.
The RP-21 are a little smoother, with a greater quantity of bass. The HD280 has a more expansive soundstage and resolve more details. For a richer, more satisfying sound, I would go with the RP-21s.
Be sure to get some vinyl or leather replacement earpads for the RP-21, because the stock earpads are much too shallow for comfort, especially if you wear glasses.
Let me know if you would like to know anything else.