Originally Posted by VFR

If you are in Main Cabin, the DL 764 for the 2-3-2 layout. I haven't flown VS Premium or Upper Class, but I have read the 350 is nicer than the 764.

VS-marketed flights earn RDMs and MQDs according to distance flown while DL-marketed flights earn according to fare paid. I'm guessing you are flying ATL-LHR-ATL on the nonstop, so here's a list of the breakeven points for each fare type:

Basic Economy: Book VS flight numbers since DL-marketed basic fares don't earn SkyMiles or MQMs/MQDs.
Economy (VS N, O fare classes): Book VS-marketed if your fare + surcharges is less than $421, otherwise DL-marketed.
Economy (VS M, E, Q, X fare classes): Book VS-marketed if your fare + surcharges is less than $842, otherwise DL-marketed.
Economy (VS R, L, U fare classes): Book VS-marketed if your fare + surcharges is less than $1684, otherwise DL-marketed.
Economy (VS Y, B fare classes): Book VS-marketed if your fare + surcharges is less than $2105, otherwise DL-marketed.
Comfort+ / Economy Delight (VS V fare class): Book VS-marketed if your fare + surcharges is less than $2105, otherwise DL-marketed.
Premium Economy (VS any fare class): Book VS-marketed if your fare + surcharges is less than $2526, otherwise DL-marketed.
Business Class (VS any fare class): Book VS-marketed if your fare + surcharges is less than $3368, otherwise DL-marketed.