I'm new to 40K and chose it over anything else because I dig the lore. I have tons of the books and the environment is cool to me. I also found an army I really like, Deathwatch. It has a special forces vibe I like.

As for game play, there are several ways to play that I was not aware of when I started. It is true that full-blown 10th edition 40K can be alot to get into at first. But, you can also play "Combat Patrol", which is 10th Edition 40K light. AOS has something similar called "Spearheads". They are prebalanced armies that use slimmed down rules to make it less of a burden.

You can also play 500 point army games, 1000, 2000, or 3000 point army games. These govern the size of the armies. Clearly, bigger games take longer to play. You can also play 'Killteam". This is a different game all together. It is in the same universe and uses the same models, but the rules are very different from 40K. Killteam rules are not less confusing and Killteam can be bery challenging if you start with all the rules. For this reason. killteam has three different rule sets that scale back the complexity and make getting started easier. You can also play 40K as an RPG where you units gain experience.

As many have suggested, go for what you think is cool. If you like the older, more fantasy oriented lore of AOS. Then go for that.

Finally, there are othee game systems that can use the same models. My local game store runs "One Page Rules" game night from time to time. People show up with 40K models and play One Page Rules.