I like sparkpeople.com, although I have never tried fitday until now when I see this posted here. I think both are similar though! Sparkpeople does track weight and charts if you want (its under the reports section). I like how I can make many different nutritional goals on sparkpeople, and track my water consumption very easily, since I lack in that area! I think like Jenn said, as long as you write it down, thats whats important!!
I didn't know it had an iphone app, what is it called??