usb backups vs cd backups

For example windows 10. Is it more secure to obtain a CD based backup cd or a USB based backup CD as you can probably buy both currently. The thing I'm afraid of, is that the usb data is easily alterable... or is it not? I have huawei modem with some files that I can't modify, but is it a hardware feature of the given usb stick or thats just a simple software trick that can be easily broken by a potential "hacker" or malware?

Hello, 

Do you mean a media containing the OS installer provided by your computer manufacturer  (OS + drivers+ applications)? or just a media containing Windows 10 and provided by Microsoft?

Theoretically, a CD is safer since it is written once, but an usb is more practical and unless you are targeted (someone stealing, altering and then putting back the usb in your pocket without you noticing it; very extreme situation... ), i prefer the USB , faster and more reliable.

Yes I'm not sure if microsoft backup usb sticks are write-protected. Although what I am assuming is that the attacker had no physical access to the stick (for a 90-95%) but might have through a for example RAT software

Well, if you want a USB-based media that doesn't involve an external platter HDD or SSD that you can definitely write protect then you should take a look at SD media.  Whether conventional SD (which has a lock slider right on the card) or microSD (which has the same slider on the SD adapter, but not the microSD card itself) you can prevent write access at will.