It is a very rare occurrence that an infringement goes from infringement straight into litigation. There is always a dance that goes with it, usually starting from the side of the infringed party. From there discussions take place, and usually resolution is achieved. If you have a hammer knowing that you can invalidate their patent by showing prior art, then I'd probably just operate under the radar and hope it goes unnoticed by them. In truth, there's probably a good chance that their patent won't actually get registered due to the prior art so it may be a non-issue in the first place.