Both boats have a 21 degree deadrise, though the Steiger has an 8'6" beam while the Parker has a 9'6" beam.
The Parker has less windage due to the lower roofline and step-down helm area, while the Steiger has a flat floor and more windage. Not sure what the headroom is on the Steiger, but the Parker was designed for Linwood, and I believe he is 6'4" tall.
The Parker can be had with a bulkhead and a locking cabin door as well as an open-back cabin with drop curtains. I believe that the Steiger can be had the same way as the Miami model is open back, and the Chesapeake model is closed.
Those are the only facts I know of. I have not ridden on the 26 Steiger, but It is apparently a popular model for them. The 25' sport cabin Parkers are their most popular model as well, so it comes down to actual experience on both boats.
Tough choice.
You might want to PM member Huggs Tug as he recently bought a Steiger 23. He is the only CP member I know who has first hand knowledge of the brand.