I think the best choice between the two probably depends on your clients' individual situation...
If you're treating a genetic woman or a trans-woman for testosterone induced hair (typically facial hair), then you might like to make sure that she's got her hormone levels fixed first. (I'm told that the hair is much tougher and harder to kill with high testosterone levels).
The density of the hair is another factor, I think... Lasers zap all the hairs in a certain area, whereas with electrolysis you treat individual hairs. I've found that when the hair density is high, if you skip over hairs and keep the treatments about 1/8th inches apart, then you reduce the abount of swelling. You can't do that with lasers! So for example, you might suggest that you thin the area first with e/l, then use laser, then tidy up any loose ends with electrolysis again.
And of course the client's budget is likely to be a major factor!!
Lasers treat bigger areas, so any after effects will be more noticible, and potentially harder to hide. Again, that might be a consideration.
Your client might not like the idea of having to wait between laser treatments while next batch of hair grows through.
Skin color is probably important, as well. Some lasers won't treat people with darker skin tones so well.
And, yes, laser leaves the blonde and grey hairs untreated, as they don't have any pigmentation to absorb the laser energy. (At least, I've not heard of any lasers that treat them.)