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Apart from being able to put it different SIM cards what is difference between the two.
I know Vodafone start is embedded in there and cannot be removed as well but as I have never had a unbranded phone I don't know what the difference is
I can imagine it being a little faster at chewing data
I choose unbranded for....
No network bloatware.
Being able to use any SIM card.
Firmware update and brand new firmwares can be faster as they are not controlled by the network for testing before allowing the manufacturer to release it.
When coming to sell the phone it can bring more money.
Current Phone > Samsung Note 10+ 5G _256Gb > Model: SM-N976B.
Samsung One Ui 2.1 / Android 10.
Samsung Gear s3 Frontier Watch.
Samsung Galaxy Buds.
As far as I'm aware, Vodafone Start is just a portal to their other apps. I'm not aware of it using data when it's idle, but you can always look at your data usage (see under the device's main Settings menu and Mobile Data). You can probably also disable the app in Settings|Apps if you really want to. The same would apply to any other network apps.
For me, the main advantage of an unbranded device is that it's likely to get OS updates quicker and may also be supported for longer. Network apps are relatively small and the space they occupy isn't a major concern unless you're really tight on storage. The OS and Google apps take up far more and most of the latter are non-uninstallable.