The only difference between Neato XV-21 and Signature Pro are the color and the XV Signature Pro has one more High-Performance Filter.
Everything else is the same, no difference in hardware and software.

Neato XV-21 vs Signature Pro
images from manufacturer

Does the Neato XV Signature Pro have A New Motor?
* If you did a search online, you might find that there is info online that states the XV Signature Pro is equipped with a more powerful motor and picks up more dust than the Neato XV-21.
However, you cannot find info about this on the Neato official site.
* The reply from Neato Support saying that the XV Signature Pro and XV-21 are virtually the same except for the color and the extra High-Performance Filter included within the XV Signature Pro.

What about the Performance?
Did you thought that the Neato XV Signature Pro is better than the XV-21 like many others?
Wheather it has a new vacuum motor or not, the performance is more important than that.
* According to reviews from users who own both models, these 2 Neato robot operates exactly the same and they pick up the same amount of dirt.

P.S.
There is also a $20 MSRP difference, the XV-21 MSRP is $429 whereas the XV Signature Pro MSRP is $449.
For those who suffer from pollen, mold, dust-mites, and similar allergies, both the Neato XV-21 and XV Signature Pro could be good tool for allergy control as well as pet hair removal.