Anything flying near my airfield gets my QNH, because that's what the traffic in my hold is flying on and therefore separation is based on that. I'll give the RPS, but just before the aircraft goes off my frequency and is heading away from me. The airport to the south of me, with it's lump of associated class 'D', operates on QNH. I wouldn't dream of working an aircraft flying towards them and giving them the RPS because it causes too much confusion - QNH is best, always
CM