Virgin miles may seem like a great idea but remember that they will not qualify you for Delta medallion status which gets you a shot at free domestic upgrades. If you do not qualify for Delta medallion or have problems getting upgrades out of Atlanta (I'm gold and do fairly well there) then that may not matter. If you do not fly that much, it may be hard to amass the needed 90,000 miles for an Upper Class ticket to London or 190,000 miles for other locations like Japan or Africa by flying alone. Your Starwood card transfers 1 to 1 plus the usual 5,000 mile bonus for every 20,000 points transferred.

I have used Delta miles or better yet Starwood or American Express points to fly to many markets not served by Virgin. However, it partners with other airlines so that may not be an issue.

Anyhow, I'd finish my 25,000 miles with Delta first. I know people complain about how hard it is to use points but on standard miles my wife is flying SDF to LGA for a long weekend and we are flying FC to LAX through ATL.