I have used Diet Power since the beginning of March, and I love it. FitDay's ok, but Diet Power ROCKS!
I used to use FitDay, until I discovered DietPower and I FAR prefer it. I've spoken about it here and elsewhere, so if you've already heard me talk about it, then stop reading now. It is not free after the first 2 weeks (after the first free two weeks, it's like 55 bucks to own the software on a disk - so you can install it on more than one computer and then transfer info back and forth between them either via disk or email in order to remain updated) but you don't have to be online, it does SO much more than FitDay....and it is much easier to use in many different ways. It's won a bunch of awards, and has been recognized by some different health/fitness mag's and organizations, and has a forum and great technical support.
It's not just for logging and tracking your nutrition, but in doing both, it tracks more and gives much more information than FitDay. It tracks your metabolic rate, adjusting daily, based on your weight, exercise, and caloric intake, among other things. (A note to those who wonder how it calculates the metabolic rate, check it out in the Help section and it's totally explained.)
You can set weight loss goals and it projects your anticipated results in comparison to the goals, complete with graphs.
You can customize your exercise, adding exercises that are not in the database, and it modifies the calories burned as you lose (or gain) weight.
In addition to having a huge database of foods and the ability to customize foods, you can create recipes, then indicate how many servings are yielded, and it breaks down the micro- and macronutrients for each serving. You can keep a section for favorite foods (and vitamins, etc.) and then just drag and drop the items into the new day's entries. For instance, my breakfast is a combo of Kashi Go Lean, All Bran Extra Fiber, Protein Drink, and Fat Free Milk; I entered them all in a recipe I named Kashi Breakfast, put it in my favorites, and now all I need to do is drag and drop once.
It has calendar, calculator, and journal functions. It has a huge library of information on different nutrients, including which foods are rich in them. It tells you where your nutrients are lacking (based on customizable nutritional goals -- like for instance monitoring your sodium or sugar) and then makes recommendations for foods you like based on your own previous food entries.
It has so much more that I couldn't possibly go into here. Suffice to say it rocks, and I just love it!!! FitDay is cool, but DietPower is just in another category altogether.....another level of product. Also, they offer discounts on different products -- so I got my Tanita food scale and body fat scale, as well as my pedometer at a good discount. I would have gotten those things anyway, so I sort of mentally applied them against the cost of the software. For me, the cost has absolutely been worth it.....I think of it as the cost of a decent dinner out with my sweetie, and so spending that money in this way is certainly worthwhile.
If it's a matter of ability to pay, then that is a reality that noone can argue with, and FitDay online is ok for what it is and could serve you just fine. If, however, it's a question of Diet Power being interchangable with FitDay -- well I believe that anyone who says it is has either never used the product , or just didn't use it to its full capability or capacity.
These are not the same product -- not by a long shot.
Good luck, whichever route you take.