Personally I'd go AWD for the extra performance. If that isn't a factor for you there are other reasons:
I've not driven the FWD NX but 240 bhp just through the front wheels means it will struggle for grip on occasions if you put your foot hard down, and I suspect you will also get some torque steer. Moving 50 bhp to the rear will help here.
The AWD helps with traction and stability, it isn't just about acceleration.
The AWD system isn't a mechanical one with a centre diff so keeping all tyres at a consistent thread depth isn't critical - you can just replace front/rear tyres.
VED isn't based on emissions so there isn't a penalty here.
With the previous generation NX the FWD model was just an entry level grade so hardly any were sold. With the current generation you can get FWD across multiple grades but it still sells in very small numbers - less than 1 in 20. I suspect if you order one it will definitely be built to order with a long waiting time, whereas popular options are sometimes pre-ordered by Lexus GB to reduce wait times. Given the lack of popularity I'd also question resale value on the FWD models - although if you are purchasing on PCP or leasing this isn't a concern.