If you are truly interested in weight loss, and chemical details of your food choices, I would go with FitDay.

It tends to be more liberal in calculating carbs, due to the built in bias for low fat eating plans, IMO. But that results in better results for those of us who are following Low Carb instead, and trying to minimize carbs, especially the hidden ones..

And, unfortunately, the Atkins site has an inherent conflict of interest with promoting fairly high carb products versus encouraging maximum, rapid weight loss. I think that they are still very good on the theory and the philosophy of low carb, but lots of the Atkins revenue comes from selling products that have many hidden carbs, IMHO.

Stick with whole foods, that you prepare on your own, and you will do noticeably better with the Atkin's Diet.

Just my 2 cents.