This might be a really stupid question but I just bought a Samsung QE65QN95A and was wondering if their is any difference picture and sound wise between using the apps in the TV or using the apps on my Fire Cube ?
Might be.
Might not be.
You'll need to 'suck it and see' with each device and the same material from the same streaming service, probably.
(Ensuring that any 'per input' picture and sound processing settings are the same.)
e.g.
Your TV model isn't listed as 'supported' in the BBC iPlayer UHD trial (but it's not always 100% accurate). Fire Cube TV (2nd Gen) - if that's the one you have - is in the trial. So then the pictures on the few BBC UHD programmes will be very different.
If you've got a supported TV, you can enjoy our selection of programmes in stunning Ultra High Definition (UHD) and High Dynamic Range (HDR). Which devices are supported? You'll know if your TV is supported as you'll see the 'UHD' icon on any of the listed programmes.
www.bbc.co.uk
Similar things may happen with other streaming suppliers.
e.g. one device may support Dolby Atmos on some titles but the other not. (May not even be an issue if no surround system, or one without Atmos capability).
BBC iPlayer is stereo only whatever video format is used.
It's a mix of hardware and software issues and the developer priorities.
You'll maybe need to seek out the capabilities of both devices and streaming platforms via searching the internet; as I know of no single 'resource' that lists them in one place.