There’s no simple, single answer to the OP’s question since SP’s are human and therefore inherently capable of being erratic in behaviour (as are all of us). They’re not robots, and although we’d all like to be treated the same every time, sometimes it just isn’t going to happen.
There could be any one of a multitude of reasons why one punter gets better service from a SP than another punter, either down to the SP herself (having a bad day, doesn’t like the client, money/family worries, tiredness, etc) through to the punter themselves (boundary pusher, dirty, smelly breath, haggling, poor timekeeper) to the differing perception of the punts themselves (the same level of service being perceived differently by 2 different punters).
From what I’ve read from WG’s and from talking to a number of them, it is possible but unlikely (or at least, unusual) for their level of service to be higher with an “attractive” client. If that were the case, then ugly gits like me would hardly have a successful punt between us! Since I’ve had mainly successful meetings with some stunners who wouldn’t normally give me a second glance in civvy life, then I have to conclude from personal experience that my appearance isn’t an issue, either positively or negatively.