Heat's powerlevel is still unknown atm. But from portrayal, he wins in 2 things:
1) Heat is apparently the third strongest in Kidd's crew. While Sanji is at best fourth strongest in Straw Hat crew atm. Fifth strongest if Yamato ends up joining.
2) Heat is a swordsman while Sanji is a leg man. And in general, the strong characters tend to use either swords or fists. Swords such as Fujitora, Mihawk, Kizaru, Roger, Rayleigh, Oden, Ryuma, etc. Fists such as Akainu, Garp, Sengoku, Blackbeard, Whitebeard, Big Mom etc.
But who uses kicks among Top Tiers? No one. Heat has stronger fighting style.
So going by portrayal, Heat high to extreme diffs.