Hi Tom.
Although I have never punted in either parlour (as yet anyway)
from what I've heard and read from the more respected punters on this forum is that LM is more a higher standard in terms on you get more for your money style parlour with generally fitter girls who can offer more of a PSE style punt.
However, it does all depend on what type of service and girl you are after, also Cosmo, I've heard can get rather busy in peak times and you may bump into other punters in the waiting room, it depends how you feel about that too. Cosmo is slightly cheaper on the basic price but you could end up paying for a few extras whereas at LM, (I think) most of the girls include OWO in their service.
Personally my next punt will most likely be at Cosmo though, I've only had two punts and I don't think I'm filly confident enough to tackle LM or even Sandys yet.
If any of this comment is read by any of you more respected punters and it's a bit inaccurate please correct it
Cheers,
SS