PhysicsDiet has a straight line trend and pretty red/green blocks. Hacker's Diet has the more functional lines on a grey background.
Both diets use the same exponentially weighted moving average formulae.
Hacker's Diet allows addition of comments per day.
PhysicsDiet allows Body Fat %, Resting Heart Rate, Calories Eaten, Calories Burned to be recorded. PD has a fairly active 'self help' forum, HD does not and this means that HD does not have a sense of community.
Both allow Rung (exercise level) recording and monitoring other peoples progress (HD uses pseudonyms to disguise people).
HD has a yellow target weight loss line, PD does not (a major shortcoming).
PhysicsDiet has 247 users, HD has 1800+ users.
PD started in 2001 but only had more than a dozen members in 2006. HD has gained many members in 2007/8.