Actually, none are really "harder" to sex. It really just depends on the size of the tarantula when it molts as it is difficultt to tell on the smaller ones. I can ususally sex them once they get to about 2 or 3" but the molt has to to be in good shape. If you can happen to get the exoskeleton immediatley after the molt and the opisthosoma isn't torn (especially where it connects to the carapace and between the top book lungs) I should be able to do it for you. It really depends on the condition of it. They tear so easily afterwards and then once they dry, they break extremely easily.

And yes you could do it yourself too. You just would have to know what to look for. If you haven't already got it, I would HIGHLY reccommend The Tarantula Keepers Guide written by Stan and Margaret Schultz. It is pretty much considered the "bible for tarantula keepers".