Check them both out when you're there, they are only a 6-7 walk or so from each other. Personally I prefer the Terraces lounge on the right hand side (I thought it was by gate 20 rather than gate 10) as I've always found the gate 1 lounge a bit claustrophobic and I actually quite like the fake bird noises at gate 20 (strange but true). The First lounge at gate 20 is too small but the Concorde lounge is pretty decent. In general though I tend to go to the lounge which is closest my departure gate.