Both BT (Bacillus Thuringiensis) and Spinosad are not poisons. They are biologic controls for the hornworms, budworms and cutworms. They are safe for humans and beneficial insects such as honey bees, lady bugs, etc. They target only certain species and even those species must eat some leaf for the control to be effective. Contact with the pest will not do anything, the pest must eat your leaf.
By biologic control, I just mean that it does not work like a poison. There is something in it that I believe disrupts the immune system of the worms. They eat your leaf and either get a tummy ache and die, or their immunity is weakened and they die from the flu or something. It's been a while since I read the literature, but was sold on it at the time that I did read.
Spinosad lasts twice as long as BT between applications, cutting down on work and materials. This is substantiated by the literature provided by both products and my own experience. Spinosad will withstand more rain showers. (not something you'll be overly concerned with) I used BT last year and switched to Spinosad this year due to some advice by Jitterbugdude. (JBD loves organic growing)
http://en.wikipedia.org/wiki/Bacillus_thuringiensis
http://en.wikipedia.org/wiki/Spinosad