With X-Plan you can calculate in advance what your price will be, which is X-Plan price minus any available to your or your market discounts.
My view is you haggle first and see where you can get. Then if you don't get better than the X-Plan price, you drop the X-Plan on the table.
Things to keep in mind. The sales person and dealer may or may not do better with the X-Plan or other back end money they are getting directly from Ford. Based on this, they may try and steer you in one direction or the other if both are on the table up front.
I have had situations where I negotiated my best price, which was more than X-Plan, then told the sales guy, I think I'll just use the X-Plan and they came back with a better than X-Plan price and it was a significant difference.