the XL Pro is much lighter weight and better balanced dues to being designed to fit in the smaller 'SL' housing. Also due to the use of the 8-AA battery tray rather than the heavier 4-C cell battery system.
They both use the same analog display, and most of their in-field performance is similar.
The older 6000 Di Pro SL had an external Volume control, which had the factory preset setting at maximum, and that was eliminated on the improved model.
White's wanted to answer the demand for a 6000 Di pro SL in a smaller and lighter configuration. That model's electronics couldn't 'fit' in the SL (SlimLine) housing so they had their engineering team design a new model with the goal to match or better the performance of the 6000 Di Pro SL.
Speaking with one of the team that worked on that project when I was using a prototype, I asked about the trade-offs. I was told that only 15-to-17 parts were carried over from the 6000 Di Pro SL. That's not many. The analog Target ID performance was the same, and all of the mode options and other controls were the same, except they removed the Volume control. Instead, they preset the target Volume internally slightly louder than before. Also, the result of the project was a unit with slightly more sensitvity.
They share the same frequency, the same coils, and the bame basic set-up and performance. It's just that the XL Pro is the much better feeling unit and handier to use for longer periods at a time.
Monte