My experience (of about 7 females over the years)is that it doesn't make a bit of difference. We alwasy now get ours done at 6mts on the dot.They can start "calling" at this age and it really isn't good for them to have a litter before they are fully grown themselves.
It is more expensive to have a female sterilised, due to the type of operation - think about it! I think my vets charged about £40 for a male and £60-70 for a female - I think - can't remember exactly... mind you with the males there can be complications - one of mine had undescended testicles and cost a bloody fortune.
Ring around a few vets to get a price for spaying, it can vary a lot. It won't make a difference to how affectionate it is if you let it have kittens, but then you'd have to find good homes for them, and there are a lot of strays out there already.