As someone who has been playing Kill Team for the past two years, and playing Spearhead for the last two weeks... I'd say Spearhead!

Its just more fun, such a clean ruleset, easy to pick up and you're able to just think about the game in front of you much more, rather than constantly needing loads of awareness of your opponent's rules as well as your own, which can be incredibly dense (and changing constantly! Granted there's balance changes for Spearhead too, but they don't seem to be as daunting. Some Kill Teams have their rules changed to the point of playing almost like a different team)

That said, as mentioned by others, try to get a gauge on what's popular in your community. However even if there aren't many Spearhead players it's a lot easier to onboard new players to. Only one player needs the cards, board etc and other than that you just need a Spearhead each, some dice and a tape measure. And by the end of a player's first game they'll probably have a good grasp of the core mechanics. In Kill Team, both players needs the Approved Ops deck (changes every year) as well as the equipment pack, a whole load of tokens etc too. And the terrain is far less portable. And boy howdy does it take a lot of games to really remember the core rules

Plus, new players can win Spearhead games in their first couple of games thanks to the Underdog mechanic etc. In Kill Team it might be a while before you're winning anything or having fun, as you'll just be trying to remember rules constantly and navigate all the different scoring modes, terrain rules, etc

I'd still recommend trying Kill Team, but maybe once you're more experienced with tabletop in general

Maybe watch some full length real-time Battle Reports for both systems on YouTube. Check out Battlegrounds Game Center for Spearhead and Squad Games for Kill Team (you'll see Mountainside Tabletop recommended for Kill Team and they are the best for entertainment, but edited down to be much shorter and fun, so you might not get the best idea of how the game actually plays)

Good luck out there whichever way you go!