I am a 40k player. I have roughly 4000pts of Chaos Space Marines and 2500pts of Necrons. I also have a number of random models (Assassins, Space Marine characters etc) and terrain. The people I play with are equally invested in 40k and we all love the game.
So naturally, I voted Sigmar - and here is my rant why.
First and foremost, Lizardmen are my first true love. Yes, they're mostly unassembled and I've never fielded them before but that's where my passion lies (and I probably have more Lizardmen models than 40k combined).
Secondly, I've lost the arms race in 40k. My army was built around the 4th edition rules and I've struggle to keep up. Nowadays with flyers and superheavies a constant threat I just don't have the models to deal with it, and frankly if I did it would be a super boring and non thematic army. The new army composition rules (unbound/battleforged/formations) to me are confusing and make army list creation and optimisation incredibly time consuming and frustrating, not to mention that some factions get far superior formations or benefits than others. My poor old Combined Arms Detachment of infantry based Word Bearers has lost the game, and the world has moved on without me. I still love 40k and still play with my friends, but I have no intention on branching out into a world where I can be curbstomped by a Tau gunline with an Imperial Knight or whatever next month's filth is.
Third is time. I have a little girl now who turns 1 tomorrow. I love her but dear god my hobby time has evaporated! Since she was born I've had 3 gaming days with my friends and painted 2 models. Yeah I've got a lot of painting to get my Lizards up to scratch, but AoS has speed of play as a huge drawcard for me. I don't need to write an army list and I don't need to optimise my army. Does my opponent have a harder list than me? Cool no problem, I'll plop another unit of Saurus down. Balanced yet? No? Ok then, more Saurus (I have a lot of Saurus). The core rules are 4 pages long as opposed to 80 or so, surely I'll be able to pick that up pretty quick. All the unit rules are right there in black and white on the units warscroll, no longer do I have to search through a codex or two and the rulebook to determine exactly what a rule does or how a unit interacts with something. In the time it takes to plan, set up, deploy and play a game of 40k I reckon I could fit in 1 or 2 similar sized games of AoS.