What happens if we choose both Incremental and Block Level Backup in the same plan? First, before I answer, I want to say the team changed terminology recently in what these backups are called and it's confusing to many.
Second, you do not need the block-level backup option if you do not want it. All block-level does is back up all changed / new files and may back up only the changed portion of larger files. If you do not have large files that change in a small way or you'd prefer to back up all files that change in full every time, then do not use the block-level option.
Third, if you decide to use both backups because you have large files like Outlook PST files that change little day-to-day, but are large, and you do not want to back up the entire file every time it changes, then do not schedule both job types at the same time (there's no benefit). Schedule the incremental, say every 14 days, and the block-level every day. Your actual schedule may depend a lot on your retention settings. When the block-level runs, all files that changed will be backed up - but some may just have their internal file changes backed up. When the incremental runs, all files that changed will be backed up in their entirety.
If you wanted to keep 14 days of backups, but used a 30 day Incremental Backup schedule and daily block-level backups, you'd have to store about 44 days of each file before the original backup set of 30 days of that file could be deleted from backup storage. Instead, you'd be better off using a 7 day incremental backup schedule. That way, after 21 days, the first 7 days could be removed, leaving you with 14 days of backups. Feel free to post your actual retention needs if you have questions. Or, if you don't have large files that would benefit from block-level backups, then schedule the incremental daily or more frequently as needed.