Well, Massive Feet it is the best just for saying. Iron feet it is a decent leg to affect physical but does not help with mech resistance. You are looking to build a heat mech having Dynamite Boots. Dynamite it is an ok leg been functional depending on strategy for the leg function having a 2 knock back when hit the opponent with leg. The HP it is decent because it is close to same average with other legs and affect just a bit to overheat and heat damage it is average. It does weight close to the others.
Now, if you use Scorching feet, will knock back only one like Iron feet but will help with heat damage and overheat too to help your heat attack in case you need normally been trapped. Sadly, not resistance damage but the weight it is less than Dynamite and can use that extra weight to enhance modules or even add a better weapon. The legs are obtained in common and rare basically on every farm you do and collect boxes. It is not in use commonly now, but it is a leg that go to Myth-Divine status. In the future will have to change to Massive.
So, Scorching can help like the Dynamite close on damage but Dynamite help to push more but it is heavier on weight.
All depend on modules available and weapons at the moment what better will suit you to use. Your inventory will change often and will have to re-adjust for it.