You don't actually get any increase to SrS damage uniquely as SV until 70 (Imp SS) and 80 (mastery). So it doesn't really matter. My guess is BM would probably be best as it gains it's buffed AoE the earliest (Beast Cleave @ 24) whereas marks gets Bombardment @45 and Wild Quiver @80, and SV gets Serpent Spread @68. Once you hit 68/70 SV likely takes over as strongest AoE.