Depends on your use case, if you have Amazon services FireTV is probably the way to go.
On the plus side it is built well, and pure android with updates (most other android devices do not have updates).
On the down side apps are limited, after using the Android TV (leanback launcher UI and remote), which comes with the Nexus Player, I would prefer a full android OS with all the apps that go with it. You need to add some additional things like a USB hub, mouse and possibly a keyboard (although not required). This is not difficult to add but takes some doing.
There is a new rom available called lollirock which puts a full android 5.0 build on it and I am looking at that.
Overall I am happy with the Nexus Player and will likely put the new rom on and get the full android os. What I like is that I can always go back to android TV and upgrades will be available. The build quality is also very good, compared to other android set tops I have had (mostly from asia) which cannot be upgraded.