There are quite a lot more differences than the 2 you have mentioned. Im not going to get technical cos I will be here for hours. ITF is well known to be considered the original taekwondo form, as created by General Choi Hong Hi. WTF was created by the South Korean government, when they took Gen. Choi's ideas and capitalised on it by turning it into a sport and showcase for Korea. WTF is now a sport more than a martial art for self defence (which is how it became an olympic sport)....fact.
ITF have hand & foot guards, as well as helmets and a box....how would that weigh you down when in WTF you have to wear bodyguards?? You cant say that WTF have more intense sparring sessions as each club ITF or WTF will have its own training methods. In ITF there are continuous sparring rounds.....in WTF you get stopped after every point you score!
Plus, you should see a physiotherapist about you achilles.....ive been training for years in many different arts and have never had problems, and niether have any of my friends. You should analyse you warm up techniques.
I recommend that if you want to take up tkd, you try several different dojangs both ITF and WTF and see which you prefer.