Vi would suit your playstyle more.
This is coming from a Hecarim main.
My hecarim build is:
Stalker's blade - Warrior, Iceborn Gauntlet, appropriate boots (swiftness, ninja, or mercury) then full tank (e.g. Randuin's omen, Spirit Visage, and Sunfire cape. Sometimes Zz'rot Portal or Ohmwrecker, or banshee's veil, or frozen heart, or rarely youmuu's ghostblade depending on what the team needs and what the enemy comp is). You can substitute Trinity Force for iceborn gauntlet as well (would also give you more offense to fit your playstyle). Hecarim does a great job diving carries and initiating fights, but unless you get really ahead he isn't going to 100-0 a squishy - you need your cc to lock them down while 1+ teammate(s) helps get the job done.
On the other hand, Vi with warrior and trinity force can probably take down a squishy by herself. She has better offensive steroids and burst for this purpose. Her ult is also targeted which helps.
Final note: Small PBE nerfs for Hecarim incoming. Vi just got a small nerf last patch. Basically, if you're only looking at them because they have good win rates, be aware that Vi will probably maintain a high win rate for longer. Also Hecarim actually wins more top, he's below 50% in jungle (although he's still great there, I'm starting to like running teleport+smite on him).
I think you'll be happy with either decision. Good luck!