I just beat The Evil Within an hour ago, so I can finally give an informed answer!
Both are good, but Resident Evil 7 is the superior package. That said, it really depends on what you're looking for. RE7 plays a lot like the earlier RE games, just in first person and with a dash of hide-and-seek gameplay. The Evil Within is more of an extension of RE4, with a lot more stealth and survival thrown in.
RE7 is also a scarier game. TEW has a lot more gore and atmosphere to spare but it doesn't have that sense of dread that makes RE7 so suffocating.
I'd recommend playing both, if you have the time and resources! Good hunting!