Both are top roller trainers, the Neo is the Tacx high-end product. Depends on your needs, a professional cyclist will probably opt for the Neo, the ambitious amateur will be satisfied with the Flux 2 Smart.

Neo:
+ motor brake
+ Resistance 2200 vs. 2000 watts
+ Accuracy 1% vs. 2.5%
+ Gradient up to 25% vs. 16
+ Simulation of descents as well as road surface (e.g. cobblestones)