I have a 12 yr old border collie cross, I want to get a lab pup, but which sex, a girl or a boy? we had a lab cross bitch who died 3 yrs ago.

It depends very much on how the dog/bitch you have gets on with other sexes.  If it is a dog (you don't say) then the safest bet would be to have a bitch.  If he is a laid back non agressive sort of dog, you should be fine with another dog.

If your current dog is a bitch, again the safest option would be to go with a dog.

Obviously when considering your options, if your current dog/bitch is not neutured, you will have problems when the bitch comes into season.

Hope this helps
Sandra

Opposite sexes tend to be the safest, but most males and females live very happily together.

Some questions to ask are:

Is your dog neutered or spayed (you don't say what sex it is)?  If not, and you get one of the opposite sex, you will need to consider how you will keep them apart during seasons.
Does your dog have the temperament/health to put up with a pup?