I would keep them separate. If you copy everything to the SSD and do a backup of it, everything is in that one backup file, and if that gets corrupted you could lose the lot. So then you get to the stage of having multiple backups of Windows+Data, which take up more storage space and take longer to do.
With the data stored live and separately on a different drive, if that drive starts to fail you could still access the rest of the data on it and perform recovery operations to get it back. It is far better to have backups of the stuff you just cannot lose like photos, documents, everything else can be got again off of the internet such as passwords and bookmarks, programs etc.
In an ideal world you would set up a fresh copy of windows, install drivers if necessary, install just the main programs you always use, then back that up. Then as you are using windows, you are getting updates, installing more programs etc so you do an incremental backup - now you have the choice of restoring the fresh install, or the latest incremental backup you did.
You can then do a separate backup of just your data to a completely different hard drive, ideally unused for anything else.