If its one or the other, I would go the lawn first. You can always configure a sprinker system with several pulsators in a run and have a timer on the spigot to do the watering.
On any contractor, I like it when people do their homework and ask for: references, if I will be on the job to supervise, warranty and their follow up, newest tech in equipment, anti syphoning mechanisms, how clean they will leave the job when done, and payments. Payments, I think you should be leary of contractors wanting too much money up front. I would feel comfortable with something like a third up front, a third one half through, and the balance upon completion.
So many times its this scenerio that homeowners do not like. they meet this nice person that explains the system, the procedures, the payment, the clean up, and they feel secure and good with this person: only to find out that this person will never be seen again. You have a band of strangers at your home just hoping that they will do everything that was agreed upon.