There is no mechanical difference between Neato XV Signature and Signature Pro.
The robot is virtually the same except for the color, MSRP, and the included kits.
XV Signature Pro vs Signature
images from manufacturer

Color
The XV Signature is overall Black whereas the XV Signature Pro is Black with Gray on the mid part.

MSRP
The XV Signature MSRP is $399 whereas the XV Signature Pro MSRP is $449.

Accessories
The included brush and filter are the key difference between these 2 models.
The XV Signature comes with 1 Blade Brush and 1 High-Performance Filter whereas the XV Signature Pro comes with 1 Combo Brush, 1 Blade Brush, and 2 High-Performance Filters.
With the Combo Brush, the Pro makes less noise, especially on hard floors, and the Combo Brush also picks up pet hair better.
You can buy the Neato Pet & Allergy Upgrade Kit including 1 Combo Brush and 1 High-Performance Filter to upgrade the Signature model to Pro.