What u would have done for this particular option
1tb HDD + 256gb SSD
Or
512gb SSD(440gb)
Tough call and based on how big of a data hoarder you have.
Based on this peculiar option, i'd say you have a laptop.
Now, for my own personal use, i would've gone with 256 GB OS + 1 TB HDD. Since i do a lot of photo editing and i need space to hold my projects + homemade videos and music files (hence 1 TB HDD), while on 256 GB SSD, i can house OS and 1-2 games that i play.
If you do not need loads of space, 512 GB SSD only is better option, especially since you can always add more storage later on, and HDDs are cheap.
after installing Windows 10 along with backup
Depending what kind of Win backup you do. If it's only "My Documents", then it can fit onto same 512 GB SSD. But if you do full drive image, then consider what space you've taken up, and multiply it with two. For full image backup, 2nd drive is better, since you'd need far more space.