Re: Windows or Linux?
Can you delay or disable updates to Windows IoT?
Does Windows IoT cost real money?
These two are related. There is a free "Windows IoT Core", but you can't disable updates.
If you're prepared to pay for "IoT Core Pro", you can turn off automatic updates.
There's also "Windows 10 IoT Enterprise" which seems basically to fill the space once occupied by XP Embedded and its successors. And "Windows 10 IoT Mobile Enterprise" for "business mobile devices", presumably while such devices still last. More here.
Don't know about telemetry but the clear aim of IoT core is to hook up your devices to Azure, so one way or another they're hoping you're going to be checking in with the mothership.
Whether the IoT strategy lasts any longer than the mobile phone strategy would also seem to be a concern for any potential developers.