It's a completely different thing. Single-board computer versus a microcontroller. It might not matter for some applications, but it's a major tradeoff.
But the only value of Arduino is the community (and the compile-time layer of syntactic sugar, if you like it). Otherwise, it's just an expensive breakout board for a cheap chip you can buy from Mouser or DigiKey. If you know how to solder, you don't really need the board in the first place.
First, the software is available whether you buy the board or not.
Second, there's no real difficulty barrier, not anymore. There are point-and-click tools, free integrated IDEs, cheap programming dongles, etc. There are more tutorials for Arduino than the underlying chip, and I'm not saying that doesn't matter - but it boils down to the community, not the hardware.
But the only value of Arduino is the community (and the compile-time layer of syntactic sugar, if you like it). Otherwise, it's just an expensive breakout board for a cheap chip you can buy from Mouser or DigiKey. If you know how to solder, you don't really need the board in the first place.