(note: I will refer to the stoner design family of DI rifles in 5.56 colloquially as the AR or AR Family... Meaning civilian ARs and the Military M16/4 rifle on the whole, just for simplicity and due to the fact that civilians are primarily concerned with the semi AR version only.)
Shooting an AR vs an AK... Is very different. The AK is a more visceral experience. Lots of fun, but I don't feel it is as easy and quick to use as an AR.
If I was to need a rifle for serious work, I would choose the AR over the AK.
A modern AR is very reliable... Provided that it meets a minimum standard of quality construction. Basically if it can meet the mil spec standards, an equivalent spec, or be very close to it and from a good manufacture... You are good to go.
Something like a Colt 6920, is a good example... Aero Precision makes a good rifle, but not strictly mil spec, being they use melonite treated parts and barrels. Melonite is a proven treatment for steel, that performs similarly to chrome plating. There are differences, but they are involved to get into... For the civilian using a semi auto, the melonite offers a cheaper solution but maintains a high level of performance. (like all things, it must be done right, cheap crap is cheap crap no matter what the box says about the manufacturing processes.)
Another example of a solid performing rifle is BCM. They were the manufacturer of the rifle used in the "filthy 14" mini experiment done by a big name firearm training company. (Its worth a google)
Daniel Defense is another example of solid performing rifles.
You are looking at a price range that spans $1000 difference. The basic Aero rifles going for around $600, the Colt $850-900, BCM in the $1300+ range depending on the model, and DD being $1400+ on average. (I would trust them all to work well, but I do have a soft spot for BCM, I think they meet a good balance of quality, performance and total cost.)
If you are willing to spend the money, companies like Knights, LMT, and others have altered the basic design of a few parts to improve long term reliability, and even basic day to day reliability in harsh conditions. Improved bolt lug design, and carrier cam slot changes are an example. I think they are great improvements, but at a cost that puts them outside my comfort level for cost to performance ratio. The improved performance isn't significant enough for me to justify the cost.
If you know how to maintain/clean the AR properly (and I don't mean it needs to be squeaky clean, just that you know what works and doesn't, and what is true and what is myth when it comes to AR cleaning and field maintenance) and know the proper preventive maintenance (like recomended spring replacement schedules, etc)... You will have a very reliable rifle. The military tends to dominate the conversation when it comes to these things... But much of the general knowledge of the average service member surrounding the AR platform is wildly inaccurate. Fortunately, in recent years, this is starting to be corrected by outside trainers coming in and setting things straight.
SOCOM dropped the SCAR in 5.56, and went back to a rifle based on the AR family. SOCOM which can choose their rifle, and have the funds to do so... Didn't find that the SCAR performed significantly better than the AR. The Army tested other options as well, and had the same result.
I don't go for piston variants of the AR, they have advantages, but also drawbacks. DI costs less on average and the rifles tend to be lighter, which is good to have.
7.62x39 can be pretty cheap, but 5.56 bulk is pretty cheap right now too.
I prefer the external ballistics of the 5.56 over 7.62... And the difference matters more for ranges past 250yds or so.
On average, the bulk 7.62x39 will be less well made and less accurate than bulk 5.56 is.
On average, the AK rifle is less accurate than a modern AR. You can get outliers of particularly good and bad with both, but the AR tends to be more accurate on the whole. Dollar for dollar, the AR wins more often than not, as a $600 AR will likely be more accurate than a $600 Ak.
An AK is more tolerant of neglect than an AR for sure, and it can beat the AR for reliability is some situations. I wouldn't declare it as an always true blanket statement though. It just isn't the be all end all rifle some claim.
Selecting a quality AK is much more challenging... There is a lot of cheap crap out there, with a lot of the well made stuff costing $1000+
For serious work, the AR is better IMO.
For a general purpose rifle, I feel that a 16in barreled AR with mid length gas system is the best option.
I own both, I like both... But I shoot my ARs a lot more than my AK.