I've never tried this combo but just by having Eos I think WHM/Selene wins. Eos really only gives a HOT every 1 min and a +20% healing buff every 2 min. I'm not counting Convenant as AST has Collective Unconscious and I didn't hear about any fight being a wipe without Fey Convenant (tho I might be wrong since I haven't reached A8S myself). If you're not using Fey illumination at the same time as Synastry then it's not even compensating for the burst as you will never be close to a Cure3, which is the only reason I see to want Eos vs Selene in the first place. So unless a Synastry + Fey illumination are needed at the same time, you'd better take Selene which synergizes well with the low GCD of AST.