Flying Delta Biz, US to Eu - better with VS FC# or Delta #?

Just booked a business trip JFK to MAD business on Delta, a question: should I be using my VS FC account (Au) or Delta skymiles (low status)? Which one gives me better value?

Fortunate enough to be lifetime Au, (30 years FC member now!) so... 'Tier Points' are of minimal concern, just about the miles really. Delta miles seem less valuable in $ than VS ones, but... can be of use for domestic bookings? Hmm - advice appreciated!

Surely that would depend on if your likely to fly VS or Delta domestic using points?