I'm probs qualified to give an opinion here, I have both.
In terms of performance, the Delkims are better IMO. They are more sensitive, more adaptable..... basically if you want to know what's going on at the business end in any fishing situation the Delkims will tell you, if they're set up correctly.
BUT...... the reliability is utter, utter pants! In three years I've had to send mine back 3 times for repair due to water ingress. The first time the white one went wrong after heavy rain, it started beeping intermittently, the beeping got faster and faster until it was basically just giving me one constant run and wouldn't stop. I thought that it could have been my fault (maybe not closing the battery cover fully) so after repair I was very careful with my set.
One year later the same thing happened to my blue one, off it goes for repair. I'm disappointed but by this time I'm carrying a spare old Fox Micron M so it's not the end of the world. I'll give them one last chance.
This year the same thing happened to the red on. Now I've had enough. 500 quid and they're not waterproof. Time for a change. I got home and ordered the NTXr.
To rub salt into the wound, I've had my red one back from repair with a note saying I need to now send my receiver for a software update (I cannot now pair it up with the red alarm).
The Fox's are OK. They're not as sensitive as the Delks in every situation but the build quality is a million miles better. They feel invincible compared to the Delks. Do they go beep when a carp takes line? Yes - so 99% of the time they'll be fine.
I'm currently holding on to the repaired Delks to make sure I'm happy with the NTXr, then I'll sell them. At this point I'm happy with the NTXr.