It depends on what you are trying to do, but for a general-purpose SHTF/plinking rifle on a limited budget, an AK is hard to beat. AK's are widely available, affordable, and parts are extremely common, making logistics very simple.
If you are seriously into precision/long-range shooting or hunting, an AR10 is a better choice, chiefly because it is a .308 and is undeniably capable of a greater degree of mechanical accuracy than an AK.
There are fewer issues with mag commonality in AK's than there are with the AR10, as some AR10's are configured to run 1960's-era original AR10 mags, some are configured for modified M14 mags, and others are set up for modified FAL mags.
AK mags are very common, almost a dime a dozen, and most varieties are completely interchangable. Additionally, if you break or lose an AK mag, you're only out a few bucks and you can easily get a replacement. Same deal with other AK parts.