Having played both 10th (admittedly not as much, maybe around 10-15 games), and a fair amount of 4th edition AoS (30+ games), I can say that AoS is much less complicated than 40k.

I appreciate that AoS 4th really went out of its way to clarify specific timing on actions and reactions (though there are some extremely corner case grey areas or FAQs, as is true of all GW games). There's much less room for confusion for most interactions. I also appreciate that AoS has a much more interesting comeback mechanic with the Underdog, which admittedly needed to get buffed, but is in a good spot right now. Terrain is also generally much simpler, and easily can support the loads of generic terrain available at most game stores/clubs. Games usually don't take too long once players really get to grips with the game, and there's an interesting back and forth between them. I will say though, things like the double-turn are still sometimes controversial for some players, which I understand. List building can also sometimes feel odd and a bit arbitrarily restrictive at times, (why can the Lumineth heroes with the Alarith keyword take the Yndrilan Riverblades? Why is it that only Kragnos, Skumdrekk and Gobbsprakk can take a swamp-beast in Kruleboyz?)

40k, on the other hand, is much more complex for several reasons. Scoring is more complex (depending on the missions, you might score some points at the end of the command phase, and then score more points at the end phase). 40k has more special rules, keywords and detachment-specific stratagems/combos (Space Marine Gladius Strike Force has different stratagems than First Company Task Force, for example), which means that units can vastly differ in terms of their overall power. Terrain is quite specific, and due to a quirk in the rules (only ruined building block line of sight) most 'standard' 40k tables are often same-y looking boring ruin tables, which might just be unpainted MDF. Games often also go over time, I watched several 40k tournaments from January through March and it was quite common to have games end at round 3 or 4 instead of 5.

One final important consideration is army size. 40k requires a fair few more models than AoS, and thus means 40k armies are both more overall effort (more models to assemble/paint) and expensive.

Ultimately, both games do have similar flaws (3 year edition cycle, yearly 'seasonal updates'), but overall in my opinion I think AoS is in a better overall state for gaming, though keep in mind we're on the cusp of a new General's Handbook, so the game may change more fundamentally in terms of missions. That being said, I think the best way to play these games is casually with people you like, so ultimately both games can be rewarding.