It’s typically a bad idea to get advice off rando’s off the internet. Not just for wow, but in all things in life.
Every spec in the game can be viable in 90% of the difficulty content. There might be some extreme outlier examples (sorry but I haven’t seen a single frost dk since DF launched, mages have been pretty scarce too), but even those can get key stone hero (2500 io) relatively easy nowadays.
Just like any other spec, it comes down to personal preference. You will most likely sink a lot of time into the spec you main, so above all else be sure you dont mind playing it for 20+ hours a week.
My general perception of the specs:
Vengeance: mobile, high damage, super squishy. Not a community perceived “meta tank”, so you will be declined from most keys in lfg. Going beyond the meta though, they will be fine and you can do it if you get good and have the right talents, gear, etc for the given situation.
Havoc: DF is the first expansion ever where havoc takes significant skill to play. If you want to get the most out of the spec, you will need to get used to playing momentum. I personally love the playstyle, but the community absolutely detests it. Take it for what you will; it’s a high risk playstyle that can be frustrating sometimes but is also very fun and rewarding when you git gud.
Don’t listen to the haters, havoc is a great spec and has a damage profile that can pump in almost all situations. It is a top tier dps in m+, and as of this morning, sims the 4th highest single target damage in the game after their recent round of buffs, according to simcraft.
The only real problem with the spec? You’re not a ret pally
ret pally is 1000x easier, has comparable if not better ST, and also pumps in aoe situations. It might not THEORETICALLY be the best melee spec, but community perception without a doubt says it is. A lot of m+'ers will decline you because they only have one melee dps spot and want it to be a ret pally.
tl;dr: havoc slaps, you’ll be fine if you play it, but at the end of the day it’s a video game and play what you think is FUN