If I want to get into the AR platform, is it better to practice with an AR-style .22 or should I just go right to the standard .223/5.56? I like how cheap .22 ammo is, but it isn't the same as the "real thing". Should I just buy both?
.22lr for sure...ever since the precipice of WW3 in Ukraine/Israel, .223/5.56 has slowly crawled up in pricing. It's to the point that .300blk is almost the same price.
Get both. A .22LR will not only save you money on ammo, but will also open up options like closer-range practice on steel targets and if you're dependent upon public ranges to shoot pretty much all of them have a backstop rated for .22LR, which is not always the case for high-velocity centerfire rifle rounds.
Make the .22 a dedicated rifle, don't just throw a conversion kit in your .223. .22LR is filthy and will crud up your centerfire gun, including important stuff like your gas system. The barrel twist rate of most .223 ARs isn't optimized for .22LR bullets, either, as those will prefer a 1:16 twist.