I think the approach of both products are completely different. This is from the Ultima description:
This amazing car wash is so gentle it won’t remove even the most delicate waxes, and it instantly restores the high gloss shine of Ultima Paint Guard Plus. Our exclusive formula creates volumes of suds, yet it rinses away in seconds. Plus, our special paint conditioners resist water spotting and streaking.
To me, it seems like a very gentle soap, while NXT seems that has emulsifiers to it:
Meguiar’s® expertise in ESP technology has led to the development of a completely synthetic car wash concentrate that loosens and emulsifies even the toughest dirt and road grime with xtreme sudsing action. This pH balanced, non-detergent formula won’t strip off wax protection and is gentle on all painted finishes as well as rubber, vinyl, and plastic components. Meguiar’s NXT Generation® Car Wash is the first step to an amped up shine, and prepares your paint for an application of Meguiar’s NXT Generation Tech Waxâ„¢.
My opinion is that if you want a soap that will not remove wax, go with Ultima or Gold Class, if you want to stay on the meguiar's side. If you need something a little more potent, go with NXT.