If someone is considering bookshelf speakers as a stereo pair and price is a concern, personally, I feel that starting with the standard Sierra-1 and then eventually upgrading to the Sierra-2 is the best possible route to take.
Where the Sierra-1 NrT makes the most sense is when it is being used as Surround speakers for a system with the standard Sierra Towers and standard Sierra Horizon. Having the identical NrT tweeter in all 5 speakers is a treat!
Of course, if you have Towers RAAL and Horizon RAAL, then the Sierra-2 become the best choice for Surround speakers.
That is strictly my personal take, but I do sort of see the Sierra-1 NrT as being a little bit of the "odd man out" when the intent is to use them strictly as a stereo pair. They're fantastic speakers, but if you were to purchase Sierra-1 NrT right away and then eventually upgrade them to Sierra-2, you would end up spending more money in total than if you started with the standard Sierra-1 and then eventually upgraded to the Sierra-2.
Of course, if you have no intention of ever upgrading, then the Sierra-1 NrT makes a lot of sense if they max out your budget