Here you've got me. I like both the Lizardmen and the Kharadron Overlords, although in AoS I'm a Fyreslayers man myself (Dwarfs riding big lava dragons? Yes please!). Generally though, there are many ways in which you can choose between 2 armies:
1. Style of Gameplay. KO is a primarily ranged army that can get badly mauled in close combat, while Lizardmen are an all rounder force. Choose the army you prefer the play style of and also choose the army with the play style that is easier for you. Can you restrain yourself from charging your ranged troops into combat, for example? (I can't when the game is going badly) if you can, choose KO because they have so many more ranged units. If you can't, choose Lizardmen because they have more melee units and are more likely to survive a melee (except skinks).
2. The Lore. Have a little brush up on your AoS lore about the two factions, and choose the faction whose Lore you like the best.
3. Painting. KO and Lizardmen are like chalk and cheese in their painting schemes. If you prefer painting armour, machinery and clothing, choose KO. If you prefer painting scales, horns and teeth, choose Lizardmen.
4. The models themselves. Both factions have badass models, but which kind of badass would you rather have? Imagine how the armies would fight in your mind, and see which looks the most epic in your imagination. Would you prefer to have a disciplined, cold-blooded army relentlessly slaughtering the minions of Chaos in melee with huge reptilian monsters tearing apart Sylvaneth abominations, or would you rather prefer an army of Dwarfs in diving suits descending majestically from the sky and blasting the foe on all sides before leaping from their vessels to finish the job?
If you're a new player, I would recommend Lizardmen. Lizardmen, even in AoS, are an all-rounder army that can afford to make mistakes and pull out of them and get back into the fight with gusto. They are also more customisable - you could choose a big ranged army with hordes of expendable Skinks, an elite melee army with tough as nails Saurus warriors or a mixture of both to get the best of both worlds, with giant monsters aplenty giving you a boost in both fields. As I say about KO, if they can stay out of combat, they're laughing, but if they're against a melee army that catches up with them, they will suffer. Badly. They only have two melee units and a couple of melee characters, making them more specialised in ranged tactics. I would say go for Lizardmen as your first army, as they would be more forgiving of mistakes in your first games. Maybe if you want, you could get KO afterwards, perhaps as allies to give some more ranged power, but to start with, I would say go Lizardmen.
Hope this helps...
